Board Thread:Warhammer 40k General Discussion/@comment-7232811-20130620224607/@comment-2253300-20130702185342

Horus was the Warmaster of the Imperium, basically second in authority to the Empeoror himself. He had plenty of time and room to build up support throughout the infrastructure of the Imperium,l and neither Big E not the loyal Primarchs saw the betrayal coming initially. Abaddon is already on the Imperium's shit list, and they watch for his action like a hawk. Not to mention the Original Chaos Legions have likely lost a good deal of their fighting power due to attrition. As for the Severan Dominant they aren't high enough a priority to warrant a serious military response from the Imperium, and it has been said that the war with them has been extended on purpose for political reasons. The Imperium could crush them, but they aren't because they have a use for the war.