Board Thread:Warhammer 40k General Discussion/@comment-24059665-20131122182800/@comment-6078851-20141117163326

there is mention of hive fleets fighting each other, but they are fighting at the whim of the hive mind.

say there's 2 fleets relatively near each other, 1 of which has a genetic pattern absorbed from whatever species recently nommed resulting in new and more effective bioforms. the hive mind will then have that 1st fleet attacked by the 2nd fleet, the fight makes the survivors stronger while the new genetic patterns absorbed by the 1st fleet get spread to another fleet. as tyranids procreate from bio-mass it doesn't matter how many die in the fight as the corpses get eaten essentially replacing every lost bioform (although this idea is largely just opinion, but logical).

as for the imperium's chances against a concentrated attack from tyranids, i'd have to say the imperium's odds of survival hover around the 0% mark. tyranids aren't actually attacking, in a military respect, anyone, the fleets are simply moving from one feeding ground to another. if the hive mind thought to concentrate the fleets into one military offensive against the imperium then the imperium would die, no question about it. with the exception of chaos i'd say that would be true for every race, if the hive mind actually attacked a species in a military manner with the intent of wiping it out before moving on to the next species, then every species would die. but because they are essentially 'grazing' their way across the milky way it gives every race a chance to get out of the way and potentially ally against them.