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 Infantryman wrote:

John Prins wrote:Any army that brings spaceships pretty much automatically wins. Almost every faction has orbital bombardment capability and Earth has basically zero means of clearing orbit of hostiles.

We have missiles that can get there pretty readily.

We have ballistic nukes. I'm not sure any of them can reach geosynch (citation needed), and even if they could, space platforms make really good use of lasers, because it takes missiles quite a while to get to that level of orbit. You could try retrofitting proper space launch missile with nukes, but that would probably take too long for it to matter (and your launch facilities are the first targets).

Orbital superiority isn't all that hot. http://www.projectrho.com/public_html/rocket/planetaryattack.php

M.

Actually, that article pretty much supports my position. Shooting up the gravity well sucks.

Not all races would do this, of course - Nids and Orks would drop to ground immediately because they have their own reasons, but IG/Marines/Eldar/Necrons have literally no reason to try to take the ground by force rather than using orbital superiority to force surrender or annihilate us.

JohnHwangDD wrote:

Tyran wrote:

 JohnHwangDD wrote:

Tyran wrote:

 JohnHwangDD wrote:

 Luciferian wrote:
A shotgun would have poor penetration against an armored target, and would require its operator to get into the optimum range for most of a Space Marine's weapons. Something like an M107 would probably do the trick a bit more efficiently and safely, but indirect fire support would be best.

If that shotgun is loaded with slugs, or saboted with DU cores, it's going to do (comparatively) very well against armored targets, such as a SM who managed to get in close. Given that a fair number of SMs have Jump Packs, having a close-in solution like the Pancor Jackhammer is a good idea.

And it's not like we lack AMRs & ATRs to comfortably deal with SMs at range.

But yes, a M107 would be a good choice.


Notionally, this is in the context of where we've gotten past initial contact and understand what we're up against. We've seen 5.56 or 7.62 ball ammo, and typical of SM tactics of mindlessly charging into blackpowder range, so we would recognize heavier weapons would be generally required along with very heavy close-in weapons like shotguns.

If shotguns had the power to penetrate power armor, it would rip off the arm of the poor guy firing it.

We don't have handheld weapons that could hope to penetrate power armor, because recoil and weight are an issue to humans.

That is flatly untrue. The force experienced by the firer is spread out over a larger area over a longer period of time, whereas the target receives a concentrated strike in an instant.

Is a BAR, 0.50 sniper rifle or AMR considered a handheld weapon? What about an ATR?

You need a tripod or a surface to support it to use those with any degree of success, and I'm doubtful they would penetrate power armor, as bolters are considerably larger weapons and those still need sustained fire to penetrate it.

40k military technology appears to be comparable to late-WW1 Earth technology, and their firearms are comparable to early Renaissance technology. SM power armor is probably only mild steel, which is why a standard Earth shotgun fitted with a "krak" slug would have good odds against it.

You are confusing design with material technology. 40k weapons are terribly bad designed, but you don't get kilometer long spaceships without insanely good material technology.
And the bolters are still a .75 rocket launcher and would still have an insane punch even if built with Renaissance technology (if such thing was possible).

A bolter hits like a lead ball from a musket. Which is bad, sure, but it's nowhere near as efficient as a 5.56 or 7.62 against unarmored infantry. It's fine against other guys wearing mild steel as well.

But at longer range? Harmless. You get a couple HMMVs with 0.50s, and those SM will get cut to ribbons before they their first shell scratches the paint.

 JohnHwangDD wrote:
A bolter hits like a lead ball from a musket. Which is bad, sure, but it's nowhere near as efficient as a 5.56 or 7.62 against unarmored infantry. It's fine against other guys wearing mild steel as well.

But at longer range? Harmless. You get a couple HMMVs with 0.50s, and those SM will get cut to ribbons before they their first shell scratches the paint.

Actually, a bolter hits like a grenade launcher.

Maybe a M203, if you cut the propellant by a factor of 3 or 4.

Tyran wrote:

 JohnHwangDD wrote:
A bolter hits like a lead ball from a musket. Which is bad, sure, but it's nowhere near as efficient as a 5.56 or 7.62 against unarmored infantry. It's fine against other guys wearing mild steel as well.

But at longer range? Harmless. You get a couple HMMVs with 0.50s, and those SM will get cut to ribbons before they their first shell scratches the paint.

Actually, a bolter hits like a grenade launcher.

This.

I don't quite buy the 'IoM used renaissance-grade technology'. Neat headcanon if that's what you like, but too much doesn't stack up.

Bolters are to all intents miniature RPGs.

Which does mean they would be terribly inaccurate and travel at a low velocity compared to a projectile which is propelled through a barrel. If you took the canonical technology involved in bolter weapons and applied it to an actual, real-world model the results would be less than stellar.

Oh yes as a gyrojet the bolter should be hilariously inaccurate unless it packs a good steering system.

Tyran wrote:
Oh yes as a gyrojet the bolter should be hilariously inaccurate unless it packs a good steering system.

True. But fortunately for the bolt gun, it's not a true gyrojet. And bolts have a mini gyrostabiliser that prevents them from flying off wildly.

Iracundus wrote:

 Albino Squirrel wrote:
We would win. Pretty easily. Just look how short ranged all their weapons are. Even the Space Marines and hordes of Tyranids would get slaughtered while they were running up to try and engage people in close combat.

The problem with Tyranids is you run out of bullets before they run out of bodies. Termagants for example are described as being expended on purpose to exhaust the enemy's ammunition.

Well, the original post mentioned a fleet of 50 million Tyranid creatures. There's no danger of running out of ammunition against that many. In fact, if all 50 million Tyranids dropped into the United States, the number of civilian owned firearms would outnumber them several times. They'd even be outnumbered by private gun owners. I'm sure there are many militaries that have enough bullets for that many Tyranids.

I guess I don't have a great sense for how quickly they can add to their numbers once they're on the ground. Or how quickly Orks can reproduce, for that matter. I guess that is the important thing. But if many Imperial worlds can handle Tyranid invasions, we should be able to. I mean, if a company of Ultramarines can demolish a major hive fleet, they can't be that impressive.

 Albino Squirrel wrote:


Well, the original post mentioned a fleet of 50 million Tyranid creatures. There's no danger of running out of ammunition against that many. In fact, if all 50 million Tyranids dropped into the United States, the number of civilian owned firearms would outnumber them several times. They'd even be outnumbered by private gun owners. I'm sure there are many militaries that have enough bullets for that many Tyranids.

In fairness most bullets don't hit their target, many Tyranids will take more than one hit and Tyranids are great at rapidly replacing their losses. Plus there's the issue of getting that ammunition to where it's needed.

Not to mention the spores etc choking defenders and mutating local fauna and flora which would be highly difficult to contain.

 Albino Squirrel wrote:

Iracundus wrote:

 Albino Squirrel wrote:
We would win. Pretty easily. Just look how short ranged all their weapons are. Even the Space Marines and hordes of Tyranids would get slaughtered while they were running up to try and engage people in close combat.

The problem with Tyranids is you run out of bullets before they run out of bodies. Termagants for example are described as being expended on purpose to exhaust the enemy's ammunition.

Well, the original post mentioned a fleet of 50 million Tyranid creatures. There's no danger of running out of ammunition against that many. In fact, if all 50 million Tyranids dropped into the United States, the number of civilian owned firearms would outnumber them several times. They'd even be outnumbered by private gun owners. I'm sure there are many militaries that have enough bullets for that many Tyranids.

I guess I don't have a great sense for how quickly they can add to their numbers once they're on the ground. Or how quickly Orks can reproduce, for that matter. I guess that is the important thing. But if many Imperial worlds can handle Tyranid invasions, we should be able to. I mean, if a company of Ultramarines can demolish a major hive fleet, they can't be that impressive.

The starting number of Tyranids is irrelevant, as the Tyranid ships are capable of producing far more. A single hive ship can produce billions of Tyranids, and splinter fleets are composed of dozens of hive ships.

And a company of Ultramarines is nothing more than lunch to a Hive Fleet. If you meant Macragge, what defeated Behemoth was two segmentum Battle Fleets.

Space Marine armour is mild steel? Really?

I thought it was all "ceramite" or something... like a mix between ceramic and steel or something? Plas-steel?

I know that probably sounded impressive in 1980s when it was written, is there anything better now? We have that chobham (sp?) ceramic armor on some NATO tanks now. That's meant to be pretty good.

Power armour is made of ceramite, a non-existant material that has whatever properties and strenght the plot demands it to have. It is in the same category as plasteel, armaplas and adamantium.

It is also why we will never know if we will be able to fight Space Marines. We can't possibly know if our weapons can penetrate their armour if we don't know what their armour is made of in the first place. Comparisons with autoguns and stubguns and such are not realistic, because those are made from the same mysterious far-future materials, unlike our own weapons and ammo which is made from boring real-life stuff.

TarkinLarson wrote:
Space Marine armour is mild steel? Really?

I thought it was all "ceramite" or something... like a mix between ceramic and steel or something? Plas-steel?

I know that probably sounded impressive in 1980s when it was written, is there anything better now? We have that chobham (sp?) ceramic armor on some NATO tanks now. That's meant to be pretty good.

Adamantine, Ceramite, Plas Steel, Fibre bundels, black carapace, redundant organs, ultra fast congealing blood, fused ribcage etc. etc.

Apparently a 5.56mm will take this down, or magically reorging the military and handing out "special anti marine" bullets/missiles etc. would happen everywhere, as someone who works with a lot of different militaries on a regular basis, this is pure dreaming, "admin up there arse" is a common phrase lol

The shock alone would have an immense effect. Plus, who cares if there are resistance movements? Losing central coordination and intelligence means the effectiveness of the military's combat effectiveness would plummet. They could also easily destroy satellites, communication towers, electrical grid, etc. People are assuming they just mass up for a battle and wait in a field for a set-piece battle. Again, that doesn't even account for psykers, Ork Rocks plummeting to the planet, Tyranid mass micro-spores blanketing the planet, etc., etc., etc.

 Albino Squirrel wrote:
[
Well, the original post mentioned a fleet of 50 million Tyranid creatures. There's no danger of running out of ammunition against that many. In fact, if all 50 million Tyranids dropped into the United States, the number of civilian owned firearms would outnumber them several times. They'd even be outnumbered by private gun owners. I'm sure there are many militaries that have enough bullets for that many Tyranids.

Look at the number of bullets fired to casualty (not even kill) ratio of most military forces. Tyranids (and orks) you have to kill or they keep coming. Discipline in the face of a horde of alien death bio-machines is going to suck pretty badly.

Also, lots of sweet sweet, biomass to convert into more Nids.

 Grumblewartz wrote:

Spoiler:

 Ynneadwraith wrote:

 Grumblewartz wrote:
People who don't think a space marine chapter could conquer earth are vastly overestimating our will to resist and never account for the more esoteric weapons and tools at their disposal. Their fleet assets would annihilate the planet. Even without using the guns, they could use the teleportation bays to jump in, assassinate leaders. Rinse and repeat. Or Psykers using mind control could just force world leaders to launch nukes at each other. Etc., etc.

Not that I'm being all 'humanity...feth yeah!', but it might be worth reading up on guerilla fighters such as the Viet-cong and a little about the generations-long conflicts still raging in the Middle East before you decide how far peoples' will to resist will take them.

An alien force zapping in and assassinating leadership is likely to do little other than martyr them. It's a viable tactic when you're talking strategic generals, but fails miserably when it comes to charismatic politica leaders.

Only way would be to use their influence to goad locals into fighting for them.

The shock alone would have an immense effect. Plus, who cares if there are resistance movements? Losing central coordination and intelligence means the effectiveness of the military's combat effectiveness would plummet.

Again, this assumes that some sort of off-world force would have the information available to them to know where and how to attack command and control centers, particularly right off the bat, which probably wouldn't be the case. Pyskers can't just sit up in orbit and divine everything or mind control people, teleporting Terminators into an office building where they're too big to fight is likely to see them rematerialize into a cubicle desk and the ceiling at the same time

Likewise, there are all sorts of examples of invaders being unable to pacify lands or being thrown out by unconventional forces. The US decisively defeated the Iraqi military...twice, and that hasn't pacified the country. Afghanistan never had anything to even remotely rival the US, and resistance movements there have yet to be crushed (as happened to the Soviets before that, and the British before that, and dozens of others before that). The Vietnamese lost every major battle they fought with the US, they never once defeated US forces in the field, and ended up winning in the end.

More to the point, modern states and military forces are extremely difficult to destroy. Little is so centralized that a single strike can utterly cripple them, redundancies are worked in, there are clear lines of succession, etc. If you wiped out Washington DC tomorrow, the US military would not be incapable of fighting by any means.

Even more important, a chapter of marines can hold...almost no ground. The entire assembled chapter could hold a line a few kilometers long at best, they'd essentially have presence nowhere while their foes would be literally everywhere. One can look at examples of the Eastern Front in WW2. Yeah, it's cool that those Tiger battalions racked up huge kill counts. Didn't matter in the end though when there were only a hundred at any given time along a thousand kilometer battle line and the Red Army simply fed them enough to pin them down and just bypassed them and shoved a thousand tanks through the gaps in the line otherwise. Rack that numbers gap up multiple orders of magnitude across the entire planet and the Space Marines would struggle to secure a medium sized town much less the entire planet.

They could also easily destroy satellites, communication towers, electrical grid, etc.

Such things are either possibilities today (satellite destruction) or are longstanding traditions of modern warfare. Nothing new to be expected there.

People are assuming they just mass up for a battle and wait in a field for a set-piece battle.

Except that this is what they do in 40k like...99% of the time.

 John Prins wrote:

 Albino Squirrel wrote:
[
Well, the original post mentioned a fleet of 50 million Tyranid creatures. There's no danger of running out of ammunition against that many. In fact, if all 50 million Tyranids dropped into the United States, the number of civilian owned firearms would outnumber them several times. They'd even be outnumbered by private gun owners. I'm sure there are many militaries that have enough bullets for that many Tyranids.

Look at the number of bullets fired to casualty (not even kill) ratio of most military forces.

Hrm, this needs to be put into context. Most of that is used in suppressing fire, where dozens or hundreds of rounds are expended to make sure someone can cross a street safely, or where a thousand rounds end up being thrown at a single story of a building because one dude shot out of a window. If we're talking about a literal wave of alien bodies advancing without care for their lives, that's going to be much less of an issue.

Tyranids (and orks) you have to kill or they keep coming.

Such forces have existed before. Melee oriented opponents of iron discipline and utter fearlessness.

The machinegun, breech loading artillery, and modern rifles obliterated them utterly. That's how you get 27,000 dead Frenchmen in one day, or Japanese Banzai charges that result in nothing more than a mound of corpses in front of machinegun nests.

Discipline in the face of a horde of alien death bio-machines is going to suck pretty badly.

Not if they're doing their best to run headlong into machinegun fire. We can look at real world examples, Musth-enraged bull Elephants don't do terribly well against poachers with 60 year old Kalashnikovs.

OgreChubbs wrote:
Nope no chance, we have no way to counter but sit here and yell at the sky.

We have gak tech to get to space and as soon as someone would make one able to get far the other country around here would turn on each other because they are getting ahead of us. Long story short we too dumb to do anything we less then orks. Hell even they can work together.

Rebels have been able to stop modern armies without recourse to air assets. I'd proffer we would be able to do the same.

Orks: They have no logistics. We would murderize them. The survivors would be a nice addition to our cuisine to supplement truffles. WWI guard beat them all the time.

Nids: Without resources it would be a blood bath but we would win. 50mm is not enough to stop us. They would run out of energy quickly.

Guard: Their tech is primarily the level of the average lower class 3rd world army. We would replicate the few higher tech items and would be an Evilz DAOT level opponent in 10 years. The, being the predators we are, we would go pick a fight with them...

Marines: Without the Holy Guard they would not be able to swoop in and take credit for another victory. They might kill a few leaders but then what? We have a 5,000 year tradition of guerilla movements. They would just piss us off. Then we would steal their tech as above.

Space Elves: Humans don't worry about no steenkin space elves.




Automatically Appended Next Post:

 JohnHwangDD wrote:

IandI wrote:
One chapter of Marines.

Have them drop them in the middle east, where so much of the earth's energy resources are generated...

As none of them fire weapons that outrange blackpowder, that'll be 1000 dead Marines in short order.


Automatically Appended Next Post:

 Kroem wrote:
The established background often has Space Marines at sub-chapter strength pacifying entire worlds of humans,

And yet, they couldn't even defeat the weedy Tau...

Space Marines only beat pre-agricultural armies.

Exactly. They may cause damage but they can't hold anything. If they stay on the ground for any period of time, even your basic peasant with an AK or red state truck driver with an AR is going to shoot the daylights out of them.


Automatically Appended Next Post:

 Albino Squirrel wrote:

Iracundus wrote:

 Albino Squirrel wrote:
We would win. Pretty easily. Just look how short ranged all their weapons are. Even the Space Marines and hordes of Tyranids would get slaughtered while they were running up to try and engage people in close combat.

The problem with Tyranids is you run out of bullets before they run out of bodies. Termagants for example are described as being expended on purpose to exhaust the enemy's ammunition.

Well, the original post mentioned a fleet of 50 million Tyranid creatures. There's no danger of running out of ammunition against that many. In fact, if all 50 million Tyranids dropped into the United States, the number of civilian owned firearms would outnumber them several times. They'd even be outnumbered by private gun owners. I'm sure there are many militaries that have enough bullets for that many Tyranids.

I guess I don't have a great sense for how quickly they can add to their numbers once they're on the ground. Or how quickly Orks can reproduce, for that matter. I guess that is the important thing. But if many Imperial worlds can handle Tyranid invasions, we should be able to. I mean, if a company of Ultramarines can demolish a major hive fleet, they can't be that impressive.

I have images of good old fashioned Texas snake roundups but with nids. Its a Texan's dream come true.


Automatically Appended Next Post:

Bharring wrote:
Also, lots of sweet sweet, biomass to convert into more Nids.

Thats only after they've won.

Only after they've established localized dominance, not wholly won the conflict.

If they drop down on entrenched military, we might stop them. But if they drop down on the midwest pre-harvest? Or Mecca mid-pilgramage? They'll face only token resistance at first, eat everything nearby, and spit out more doods.

Also, they have tools for machinegun nests. Carnifexes, Zoanthropes, Venom Cannonts, etc. 50mil Nids doesn't mean 50mil Gaunts.

Bharring wrote:
Only after they've established localized dominance, not wholly won the conflict.

If they drop down on entrenched military, we might stop them. But if they drop down on the midwest pre-harvest? Or Mecca mid-pilgramage? They'll face only token resistance at first, eat everything nearby, and spit out more doods.

Also, they have tools for machinegun nests. Carnifexes, Zoanthropes, Venom Cannonts, etc. 50mil Nids doesn't mean 50mil Gaunts.

Thats not how it works. They don't have digestive systems. Thats only the lattter stage of a Nid harvest. Also hitting mecca would put them in striking distance of 3-4 of the strongest militaries on the globe, not to mention the Rosskies and the US.

Most of the replies here are pro modern human dominance against an OP modified 40k invasion; I think we’re on the losing end on most of if not all of the scenarios.
Orks: they’re stronger, everyone is a warrior, multiply quickly; we might find a way to combat their multiplying fungal ways but would still have to deal with all the warriors already present.
Necrons: every regular warrior is a ‘terminator’ or better, good luck!
Tyranids/gsc: we die from terror alone
Eldar/dark Eldar: out teched, terror would subjugate everyone
Tau: watch gundam cartoons, suits would dominate, out teched and sly by the ethereal propaganda
IG: might be able to compete, scavenge enemy weaponry and develop tech might give us a chance, i’d give us no better than 40% chance
Marines: no chance even against their limited numbers, they can kill us with their bare fists, every marine beyond the first is a cumulative effect vs their opposition
Chaos marines: same as above with even more terror and unpredictability
Renegades: if there was a bet where modern earth have the best chance, it’s chaos renegades; earth have better equipment for once
IK/titan: no chance, we might stretch out the fighting, but no chance of winning
Skitari/mech: out teched, not as much like by the Eldar, but we still lose.

 JohnHwangDD wrote:
A bolter hits like a lead ball from a musket. Which is bad, sure, but it's nowhere near as efficient as a 5.56 or 7.62 against unarmored infantry. It's fine against other guys wearing mild steel as well.

But at longer range? Harmless. You get a couple HMMVs with 0.50s, and those SM will get cut to ribbons before they their first shell scratches the paint.

Nope. Rogue Trader has muskets and black powder pistols at S3, so bolter hit with 33.3% more power than a musket ball

 Frazzled wrote:

Bharring wrote:
Only after they've established localized dominance, not wholly won the conflict.

If they drop down on entrenched military, we might stop them. But if they drop down on the midwest pre-harvest? Or Mecca mid-pilgramage? They'll face only token resistance at first, eat everything nearby, and spit out more doods.

Also, they have tools for machinegun nests. Carnifexes, Zoanthropes, Venom Cannonts, etc. 50mil Nids doesn't mean 50mil Gaunts.

Thats not how it works. They don't have digestive systems. Thats only the lattter stage of a Nid harvest. Also hitting mecca would put them in striking distance of 3-4 of the strongest militaries on the globe, not to mention the Rosskies and the US.

Not true, Anphelion Project has nids adapting mega quickly, if they needed Digestive systems they would make them in the second wave, other nid publications also say that they dont have any digestive system because they dont need them and will die and be reabsorbed when the starve, if nids can drop 50 mil bodies on earth, then they would keep pumping them out until we cannot win, we may beat the first few waves, after that they have adapted and were done.

Nuke the site from orbit. It's the only way to be sure

 Albino Squirrel wrote:

I guess I don't have a great sense for how quickly they can add to their numbers once they're on the ground. Or how quickly Orks can reproduce, for that matter. I guess that is the important thing. But if many Imperial worlds can handle Tyranid invasions, we should be able to. I mean, if a company of Ultramarines can demolish a major hive fleet, they can't be that impressive.

If you're referring to Tarsis Ultra, you'll need to read the novel carefully. The Ultramarines were supported by the local PDF and Imperial Guard, and the Imperial Navy, and the Imperium was losing. They won only in the end because of plot armor allowing the Ultramarines to pull off a Hail Mary boarding strike at the Norn Queen's hive ship to deliver the toxin that killed the Norn Queen and caused the local fleet to revert to instincts. Before that, the Imperial Navy (and the Ultramarines' fleet) had been defeated after underestimating the ability of the Tyranids to adapt their strategy, the PDF and Guard were losing to attrition, and even the Marines were not turning the tide on the grand scale. All attempts since Tarsis Ultra to replicate or vary that toxin against the Norn Queen have also been failures, presumably because the Tyranids have already adapted to it. If one looks at the background, almost every victory against the Tyranids has been due to taking out the Norn Queen and hive ships, not actually prevailing against the main swarms.

The consumption of the planet as described in the 3rd edition Tyranid Codex starts from the moment the Tyranids land. They start using the biomass of the planet to produce more cannon fodder. So Tyranids landing in any wilderness area will rapidly swell their forces from processing any local flora and fauna.

Iracundus wrote:
The consumption of the planet as described in the 3rd edition Tyranid Codex starts from the moment the Tyranids land. They start using the biomass of the planet to produce more cannon fodder. So Tyranids landing in any wilderness area will rapidly swell their forces from processing any local flora and fauna.

Agreed. That is why the Nids landing on Mecca is among Earth's better results - it's in the middle of a frickin' desert with minimal biomass to absorb. Plus, there's at least one exceedingly heavily-armed and grumpy neighbor who would like nothing better than an excuse to glass the place.

OTOH, if they start in the middle of the Amazon rainforest, we're in exceedingly deep kimchee.

 Big Mac wrote:
Most of the replies here are pro modern human dominance against an OP modified 40k invasion; I think we’re on the losing end on most of if not all of the scenarios.
Orks: they’re stronger, everyone is a warrior, multiply quickly; we might find a way to combat their multiplying fungal ways but would still have to deal with all the warriors already present.

Sure they're strong, they also fight in ways are *really* easy to stop with modern small arms and are absolute cake for crew served weapons/artillery/aircraft/etc. It's also not like the modern world doesn't have tons of tools for dealing with fungus.

Necrons: every regular warrior is a ‘terminator’ or better, good luck!

They die just fine to all sorts of stuff. Heavily resistant to small arms fire, but not wholly immune, and small arms fire isn't what kills most things on real battlefields anyway.

Tyranids/gsc: we die from terror alone

Given all the things humanity has seen and endured, mostly inflicted on itself...probably not.

Eldar/dark Eldar: out teched, terror would subjugate everyone

Call me when Eldar tanks can match the feats of MBT's approaching 40 years old, call me, as thus far they have displayed no such capabilities. The Eldar's most advanced anti-aircraft systems that have ever been shown have basically been WW2 guns with laser beam skins, certainly nothing like modern BVR systems that can engage and destroy aircraft over a hundred miles away. Artillery is something almost completely foreign to the Eldar, especially proper long range artillery, which is what inflicts most casualties in ground based warfare.

Tau: watch gundam cartoons, suits would dominate, out teched and sly by the ethereal propaganda

Gundams are hilariously overcomplicated robots meant for rule of cool, with all sorts of (often inconsistent) hand-wavium as to why such humongous targets aren't easily taken out by long range weapons and why they need monstrously expensive legs and arms over much cheaper turrets and tracks (and why their pilots aren't turned into tomato paste when performing the maneuvers they are portrayed as engaging in, in real life their pilots would be killed instantly in half the melee engagements they're shown in from the G-forces ). Like the Eldar, they also don't really appear to have anything like modern artillery, BVR capabilities, etc.

IG: might be able to compete, scavenge enemy weaponry and develop tech might give us a chance, i’d give us no better than 40% chance

When you can actually fit into the turret and load a shell into a Leman Russ breach, let me know.

Marines: no chance even against their limited numbers, they can kill us with their bare fists, every marine beyond the first is a cumulative effect vs their opposition

Lots of things can kill a human with their bare fists, including other humans. Marines outside of infantry combat aren't all that impressive, marine tanks vs non-marine tanks aren't really at much of an advantage for instance (being an 8ft tall super soldier doesn't mean much for crewing a tank except that you probably cannot actually fit inside one). A chapter of 1000 marines could hold a small town and nothing else. You could maneuver entire armies around them and they simply couldn't be in enough places at once to stop that. They'd be surrounded, isolated, and destroyed in short order.

Chaos marines: same as above with even more terror and unpredictability
Renegades: if there was a bet where modern earth have the best chance, it’s chaos renegades; earth have better equipment for once

Chaos is about the only thing that would really have any standing here, and that's got nothing to do with battlefield capabilities.

IK/titan: no chance, we might stretch out the fighting, but no chance of winning

They'd be targeted by cruise missiles from hundreds of miles away or struck by aircraft and obliterated before they got to anything. They're big, easily seen targets that can't hide and will have gobs of ordnance thrown at them, and they're big enough that really big weapons can actually land relatively precision shots from long distances, engaging a knee joint to knock it over would be fairly trivial for tank guns or aircraft delivered munitions or even some cruise missiles. Simply *landing* a titan would be a monstrous task that could be easily engaged by all sorts of heavy ordnance.

Skitari/mech: out teched, not as much like by the Eldar, but we still lose.

Again, much like the Eldar, most of their stuff is basically re-skinned WW2 in terms of actual capabilities. Their infantry might have snazzier, more powerful guns than modern infantry, but they aren't displaying capabilities modern militaries have.

Of course, the biggest problem for SMs is that they're uniformly dumber than rocks.