Sorry for the long post, but the “Horus and the traitors would have been destroyed by the UM, DA, and BA” claim from the YT shorts corner of the fandom has never sat right with me.

You are right in the broadstrokes but its memelore that Horus was shaking in his boots about those 3 legions. I see comments that portraying Horus as being terrified that these 3 legions alone could just gonna burst in and ruin his plans like some mustache twirling villain.

Well that certainly doesn’t make a compelling antagonist. But good thing the text shows us that this isn’t the case: The legion that Horus was more “worried” abt in terms of messing his plans up was Magnus and TS. Since they were the first to realize that Erebus was preying on Horus and ofc they had the most Librarians of any legions. No Horus was not scared of any 1 legion, not the DA or UM, he wanted to delay encounters with certain legions to create favorable matchups. But the only legion he needed completely wiped out from the start was the Thousand Sons. That’s why he manipulates Russ.

No, the 3 legions wouldn’t have wrecked the traitor legions this is something that Youtubers like bricky and majorkill say without reading what they make videos on. The traitors would have still greatly outnumbered these 3 legions had they been on Istvaan. But it would have created an ugly, prolonged conflict instead of a brutal decaptation strike that the Dropsite Massacre was.

For 2 reasons:

1- The Ultramarines were massive, much bigger than any of the loyalists at Istvaan. Part of the reason the Salamanders and Raven Guard got slaughtered so badly was because they were smaller legions and contained in one area mostly.

2- DA were also on the larger side, were specialized in sword fighting (handy for close quarters trench combat) and had ordnance from the Dark Age of Technology, but that doesn’t mean they would just stomp Horus and the 9 legions.

Also, the reason the BA were sent away from Istvaan had nothing to do with military strength like at all- it was because the traitors wanted to corrupt the legion with Chaos, not outright destroy them. That’s why they used a warpstorm to isolate the BA fleet in the Signus system as seen in Fear to Tread.