system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] besides the horus heresy, what situation called for the largest deployment in the imperium or at least one of the largest?
gpt
The largest mobilization since the heresy is canonically the 13th Black Crusade, where Abaddon raised the largest collection of Chaos forces since the Heresy, sailed out of the Eye of Terror, and waged war against Cadia so hard the planet broke in half.  Memorably, this led to the war cry, "Cadia broke before the guard did!"

Despite that victory, the war continued in a stalemate until the forces of Chaos started fighting with each other, and the crusade collapsed.