Just a little gun theorising: the imperium is finally overthrown, the corpse emperor unplugged and the Galaxy under our control - so what else would we do than turning against each other.

Round 1 - Iron Warriors vs Word Bearers

Who would win this confrontation. Please try to keep personal preferences out and reason it and please don't vote without saying why you voted that way.

Edit: to narrow down the rules a little more, the warp has just begun to consume the galaxy so it's pretty much the way it is now, just without loyalist scum.

Well i voted the IV but i have to say that it really depends on how much of the galaxy has been "changed" by the warp after said victory!

Id say the Word Bearers cause they would have more favor from the Chaos Gods, and their deamonic hordes are an inexaustible resource.

Why did you have to pick two of my 3 favorites first? lol

I'd have to go with Word Bearers, because they are more organized. I'd probably pick Word Bearers winning over anything other than the Black Legion.

Really if that happened with the current state of WH40k, I think the battle would be more between Abaddon's Black Legion and Huron's Red Corsairs. Between those two I'd even bet on Huron for being more favoured.

Really don't see IW fighting WB however, but Machine >> Flesh so daemons shouldn't prevail.

I would expect the amount of equipment and fortifications created by the Iron Wariors in such an event to be more effective at dealing with a conflict than the mass cults of the word bearers, but I doubt either would go for conflict in the event of the imperiums destruction. The word bearers would likely try and return to spreading religion across the worlds again like in the crusades and the Iron Warriors would have an opportunity to create rather than destroy, something recent stories have said Perturabo was greatly interested in.

Yeah if Chaos has taken over the imperium and the warp is expanding the Word Bearers would most likely gain more from it and grow way more powerful from this than the Iron Warriors would.

My money is on the seventeenth Legion.

I think that their mortal followers come into play here. Iron Warriors use them pretty much as expendably as the Word Bearers, but the Word Bearers have the advantage here, namely in their fanaticism and the Discord. Iron Warriors mortal troops are controlled through more traditional means of intimidation, and are thus more easily broken, Word Bearers' are utterly devoted to serving their masters and will follow orders willingly to their deaths, which they can also use to their advantage by using their blood sacrifice to summon daemons.

Also their methods of control can be used to turn the enemies own troops against them, making them a dangerous foe on even both sides of a siege. Not only can they use the carnage to reinforce themselves, but the Iron Warriors' own troops will turn against them, and either by their own accord or if instructed via warpcraft, this can easily result in sabatuers arising from their own ranks.

This in combination with them being Legionnaires makes them a formidable opponent as it gives them an edge in a war of attrition, seeing as they can reinforce themselves from their enemies own ranks and from the warp.

who are the respective commander of each legion?
who will be on the defense and who will be on the offence?

A difficult choice to be sure, as despite their specialities there is a great deal of overlap between their skills. While the Iron Warriors obviously retain more heavy weaponry in comparison to the Word Bearers' more well rounded approach, both frequently use considerable numbers of cultists as fodder, both are largely united and both are shown to retain a considerable powerbase overall. The key difference stems from their connections, as the Iron Warrior retain a more pragmatic approach to worshipping Chaos but instead have much closer ties to the Dark Mechanicum. Personally, I do think this is the deciding factor, as it offers them much more overall versatility beyond the Eye.

Were the two legions fighting in the Eye of Terror itself or somewhere where the veil between worlds was thin, the Word Bearers would have something of an edge, even if the Iron Warriors would benefit from it somewhat as well. Outside of it, the Iron Warriors' heavier armour, access to Titans, and brutal pragmatism would probably help them win the day.

So, tough choice but i'm personally voting for Perturabo's lot on this one.