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What would be the largest battle in the 40k universe, specifically within the timeline of the time that the codexes are set? I assume the battle of terra would be the largest in the history of the imperium but I'd be happy to be corrected on that.

Largest in total number of beings involved, or largest size of battlefield, or most HUMANS involved? I imagine Armageddon is tops, Terra would be big though. So would be almost every Tyranid invasion. The Octarius war actually might take tops.

13th Black Crusade - although I have a feeling that this does not classify as a "battle" anymore but rather a campaign. The same could unfortunately be said about Armageddon, given that we'd be looking at an entire planet with many simultaneous fronts. This could even be said about Terra, unless we were to specifically refer to an individual engagement such as the Siege of the Imperial Palace.

I suppose we should first narrow down what we'll allow as "battle"?

Some ofthe space battles must be up there both in terms of extent of the combat volume and number of troops, given that ships seem to have crews in the tens of thousands. Some of the imoerial versus tyranid space battles perhaps?

In 40k, the battles on Cadia during the 13th Black Crusade probably take the cake. In 30k(Horus Heresy) the Siege of the Emperor's Palace is the biggest battle in the Imperiums history.

Are there any solid numbers for these battles?

Once, it was said, Tallarn was the largest Tank battle with millions of Tanks involved.


...not sure if thats modern 40k tough..since IIRC that was close to the heresy era.

Didn't Heresy Era have larger battles in general than nonHeresy Era? You'd get space marines fielded as an army back then and bane blades all over the place, etc.

Ullanor or Isstvan must have been titanic in scale, isstvan had about 7 legions i think? and ullanor had over 10 complete space marine legions against go knows how many orks. i would say ullanor must have been the largest scale battle in the history of man kind. But feel free to prove me wrong


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Ullanor or Isstvan must have been titanic in scale, isstvan had about 7 legions i think? and ullanor had over 10 complete space marine legions against go knows how many orks. i would say ullanor must have been the largest scale battle in the history of man kind. But feel free to prove me wrong

Could we class the Ultramarines battles against Hive Fleet Kraken as 'recent'? I'm not sure they're on the same level as the battles on Cadia, mind. Certainly the latter has taken longer and is still ongoing.

I'd say Armageddon and/or Cadia. Both conflicts I consider to be single large scale battles because even though there are several fronts, of course, the entirety of both planets are in conflict involving untold millions from both sides with casualties spiraling into the billions as the protracted wars crack on.

In all of the IoM's history though, nothing compares to the battles from the 30K era. The HH seems to have hosted titanic clashes that no conflict has matched since. I sooooo wish GW would advance the static storyline and introduce some insane conflicts that rival the HH confrontations.

The Octarius War, given that it involves large concentrations of the two most numerous species in the galaxy by far, probably takes the cake.

Honourable mention for Medusa during the 13th Black Crusade:
"At the height of the defence of Medusa, the gargantuan tracked fortress-monasteries of the ten Clan Companies came together to face an armoured assault launched by traitors of the excommunicated Haradni 13th Heavy Armoured Regiment. The dark, polluted landscape of Medusa was host to one of the largest gatherings of armoured might since the Battle of Tallarn during the Horus Heresy, with over ten thousand traitor tanks arrayed against the Iron Hands."

I'd say the Drop Site Massecare - 11 legions involved. gak went down on a massive scale.

Armageddon wars, 13th black crusade?


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I thought the op was modern 40k?

I second the Octarius War. It is recent (still happening) and involves two numerous races that can actually increase their bodycount during war. More biomass for the nids and more proper CC fighting for the Orks. Everybody wins!

Good call on Ullanor, had forgotten about that one.
Gets my vote.

For 10 full SM legions to mobilise (led by the Emperor) the Ork numbers/forces must have been utterly mindboggling.
And remember SM Legions were significantly larger than those of today (50k to 250k plus in number iirc).

Potentially unmatched by anything since.

I still think we need a defintion of "battle". We're straying to campaigns and events here rather than specific battles.

I'd hazard a guess that the First War for Armageddon has the largest battle. The banishing of Angron, when the Grey Knights went into the heart of the daemonic, mutant, traitor horde to banish angron.

Hordes of Daemons, Mutants, Heretics & Traitor Astartes vs Grey Knights, Imperial Guard, PDF & Loyal Astartes, all fighting along a single front.

We are totally not sticking to the concept of a 'battle'. Most of us are naming campaigns encompassing entire planets or systems. A battle is technically a single specific instance of a conflict, a war is a series of battles connected to each other.

The problem is, in WH40K, very little is small-scale, and for us to be naming individual battles is sort of hard because Codexes specialize in brevity - even the BL books I've read don't exactly get that specific with the scale of a single battle. 30K encounters seem to be a lot easier because the HH books so thoroughly describe individual encounters and all.

Can't be answered as Prehistoric points out. We only know details for a handful of actual battles, not enough to worth worrying about which was bigger.

It's reasonable to assume that any battle with multiple marine chapters must be huge with countless guard and endless enemies.

When does a battle become a campaign or a war though?

If a hive is attacked, is the battle the whole of the fighting until one side wins or is a hive so big that it's counted as a campaign. As a campaign, individual battles would be for certain levels or sections of the hive.

If just a battle, then why couldn't a planet wide conflict be called a battle if it happened all at once with so many troops that their really were no peaceful areas between the combatants? Just one huge sprawling fight.

Stalingrad in WWII is considered one battle by many historians though it took several months. I think because it was one place and continued on without stopping. A war or even a campaign in a war is several battles strung together. There's a difference. Some big wars have small battles. (Vietnam) while some small wars have big battles. (Mexican War)

 Warp Angels wrote:
I'd say the Drop Site Massecare - 11 legions involved. gak went down on a massive scale.

Certainly one of the largest Astartes vs Astartes actions, but IIRC auxiliaries were pretty few and far between so it doesn't really rank up there with battles in general, for in example the Battle of Amarah (Imperial Armour 12) the Imperials started with 12 million Imperial Guard and ten times that number of PDF, and they were decimated.

Istvaan V was marines only.
Istvaan III included titans at least, I don't know if any Imperial Army units were present.
Not that it matters as the whole Heresy War is before 40K.

The Tyranid invasion of Macragge must be high up there. Almost every ship in the ultima sector were brought together.

 Mr Nobody wrote:
The Tyranid invasion of Macragge must be high up there. Almost every ship in the ultima sector were brought together.

I keeps forgetting about this one, I know all the other ones people say but now you comes to mention it that is alot of space crafts and we all know how many successor chapters UM got. They might actully make a legion size lol but obviously no where near the old legion battles.

The thing is we always get to know how many SM chapters participated but rarely get the number of PDF or IG involvement.

Macragge was a major naval battle but as far as ground actions goes it was piddlingly small. The most notable ground battles were 'surgical' strikes against the Polar Fortresses, there was only a single company of marines on-planet when they fell, and each was retaken by a mere company each.

Im going to go with Octarius going by the Largest battle meaning the one with the most fighting bodies.

All the space marine legions togheter are still yust a drop in the bucket purely in numbers compared to a tyranid hive fleet or an ork waaagh

The Tyranid invasion of Iyanden was a doozey.

I figured it would be the battle at the Cadian gate.
how big was Medusa V I know there was a ton of different species there due to the valuable resources and the planet about to implode...

Its either Cadia or Armageddon for the Imperium. The new apocalypse book lists the Guard deployment for just one day on Armageddon at over 219 regiments, Guard forces alone. Cadia is a bit difficult to estimate, as the front was much larger than just Cadia alone.

Medusa V was listed as over a 100 regiments of guard. So even with the other forces, which dont seem that big (each their own little area of engagement), they probably dont even outnumber the Guard on Cadia or Armageddon alone.