Board Thread:Warhammer 40k General Discussion/@comment-25618639-20150604030859/@comment-5832352-20150804175845

Primarchs defecting would have still happened. I just doubt they would have all turned at once, or been a unified force without Horus. You could have had situations where up to 4 or 5 of the Primarchs would have rebelled, but there would have been little to no coordination. They might have stuck together for 1 big battle (like Istavaan), if they won, each group would have pursued their own agenda, or they would have suffered a crippling blow, and scattered. The crusade would have suffered because a large portion of the Space Marines would be gone, and now the remainder would have any more work to do as they would have had to 1) Continue Pursuing the Crusade anyways 2) Now pursue traitors

There could have been scenarios where Primarchs would have also fought Primarchs. Maybe Angron, in a fit of rage would have openly attacked Horus or another Primarch. Maybe he would win, maybe not.

You could also see scenarios where the traitor Primarchs could have turned to subtlety rather than a direct attack. Imagine Lorgar bringing the full weight of his Legion back to Terra on a "holy pilgrimage" and claiming to visit the Emperor as a sort of Penance. Then once his full legion is in place, he and his Chaos turned Legion summon daemons, etc right on top of the Emperor. Lorgar might have lost, but maybe he would have wounded the Emperor enough to still place the emperor onto the Golden Throne as a life support mechanism.