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Firstly, I apologize if there is already a topic on this. I was unable to find it with a search.

That said, I was hoping someone could help clarify some things for me. The main question is this - given some theater of war in a sector or sub-sector, like an Imperial crusade or Chaos incursion, how do the Imperial Navy and other Imperial military bodies interact? Besides the obvious of the Navy transporting ground forces to warzones, of course.

Going with the crusade example, I assume the Navy forces would be led by an officer between the rank of Commander and Admiral (based on the BFG campaign rules). Would it be safe to assume the Guard forces would be led by a General or Warmaster? If so, does said General or Warmaster outrank the Naval commander? And are there any other important leaders to worry about?

Secondly, it seem unclear how the Navy affects the taking and holding of a planet. Say we have a Chaos-held planet, defended by a traitor fleet and extensive ground forces. The Navy would spearhead a planetary assault, escorting the troop transports to their destination. If the Imperial forces are driven back, and the transports don't get to disgorge their cargo, then the point is moot. However, if ground forces land, things get complicated. It would seem that whoever has space superiority should win the planet (eventually). Say the Imperial fleet drives off the traitor fleet. Are the Chaos ground forces inevitably doomed without reinforcements?

However, it seems like this isn't usually a concern in most fluff. Everything seems to hinge on the ground battles, and fleet positioning seems to be inconsequential. Barring at least one Abnett book, there don't seem to ever be space defenses in place when a planet is invaded. It seems like ships just drop their load and wander off to let the ground forces win the day. Does anyone else feel this way?

For perspective, I'm asking because I'm contemplating some sort of campaign system combining BFG and regular 40k, and it seems that if one side were to assault a planet and land troops, then keep a fleet in orbit, the defenders are almost automatically doomed. Yet this isn't how it seems to work out in the fluff. Any suggestions/explanations?

There are all manner of surface to orbit weapons available. Large lascannons can hit ships in low orbit, Lance towers, macro cannons and torpedo launchers are all in the fluff for taking out ships in orbit.

So you could have ground missions based on defending them and if the enemy keep them then at the end of each campaign turn they can damage a ship for example.

Plus there are star ports these need to be accessed to drop the logistics down or heavy wargear. Have missions based on taking these back or capturing them. This could mean a limit on ground Armour for example

It's important to keep in mind that we only read about the interesting battles. We don't hear about the hundreds and hundreds of worlds that fell without any planetary invasion, entirely due to successful orbital bombardment. That's because for the purpose of acting out a war game, that is boring.

The battles we read about are almost never going to actually represent a 'typical' planetary invasion.

That said, surface to space weaponry are relatively common, especially on hive worlds. And as powerful as the weaponry of Imperial spacecraft are, you can get a lot more bang for your buck when the weapons are stationary. Planetary macro-cannons, missile batteries, large-scale plasma weaponry are all far superior than what you'd find on starships. This means most ships not be able to get into position over vital locations for orbital bombardment without sustaining critical damage.

Alongside surface to orbit weapons, most adeptus-level military installations would have void shields protecting their most vital areas. Similar to weapons, if you don't have to make them mobile, void shields are going to be insanely strong. Thus we can expect installation void shields to equal or eclipse titan void shields. From repeated descriptions in the lore, most void-shields are uni-directional, meaning you can have fantastic protection from orbital bombardment, but you have to rely on traditional defenses if you want to keep ground forces out.

These two factors are two of the most important aspects of war in 40k. It's why marines can remain effective despite their small numbers. If your strikeforce can take down their void shields and surface to orbit weapons, then like you said, bombard until your heart's content, the planet is already won!

As for the planet being doomed without reinforcements, remember that as a defender, it's not necessarily their job to annihilate the Imperial forces. They only have to hold off long enough and cause enough casualties for the Imperium to start to think that retaking the planet isn't worth the resources spent. Or to survive until another, larger/more important battle requires troops to be sent elsewhere. But if they're under constant orbital bombardment with no way to retaliate, then it's most likely hopeless for the defenders.

Void shields and the ability to build subterreanean cities (Tallarn, Krieg, etc.) demonstrate that even if you unleash enough firepower to wipe out all life on the surface, the enemy can still survive. More firepower is generally not always desirable unless you don't care about collateral damage anyhow, and the Imperium often does. This odes not mean that ground warfare cannot EVENTUALLY be as destructive (Vraks demonstrates that) but its more a gamble - short term probable destruction of enemy and valuable resources, or possible long term destruction from ground conflict.

But since many void-starship weapons can be, for the most part, weapons of mass destruction, that limits their utility. And even ones with 'variable yield' still will not be ideal for ground warfare (a huge anti-ship cannon is still going to be huge, and hard to manuever probably, so hitting a small, precise and/or moving target iwth a gigantic lance turret will not be easy.)

I actually suspect that lots of 'orbital bombardment' weapons may actually be the defensive turrets on starships. They're far less destructive (perhaps on par with starfighter/superheavy weapons or thereabouts at worst) and generally smaller and more precise at targeting (starfighters, again.) so they do nto suffer from the drawbacks of big ship weaponry. This can also explain why getting into the upper atmosphere/low orbit is so neccessary, because they have shorter ranges than the void weaponry.