Board Thread:Warhammer 40k General Discussion/@comment-69.143.134.163-20150419161351/@comment-132.3.57.79-20150513154955

while interesting, the idea of a 2/11 war is probably not right - in at least one place it talks about their "separate" tragedies as a warning.

when you include GW's apparent desire to blur the good guy/bad guy lines in HH (ie, each legion may have had loyal and traitor elements), I would suggest that both legions separately had infighting. To support their out of context role, it might be believed that one primarch went traitor and one stayed loayal, but the legions split. Loyal primarch probably killed by their own legion/allies (chaos) while the other taken down by the Wolves (since in several places it highlights that the only thing that usually kills a primarch is another primarch)

The warning concept also makes me think both had a run in with chaos, not a different threat (tyranid, necron).

The legions remaining loyalist elements would either be quarantined, mind-wiped, killed, or left to rot in lonely assignments never to see the world again if the imperium was true to form. Some may have augmented other units, but were never highlighted as such. There are also numerous "unknown" space marine forces operating all the way from M30-Current. Even possibly some early "black shields." Even the homeworlds might not be viable for Ultramarine recruiting - tainted by chaos they may have been destroyed or restricted.

(PS - I know, their are several variations on the "who killed the primarch" but those are either completely out of scale power or unique martyr style situations)

of course, all of this is a guess!