Maybe not the most crazy in the world's history, but once as Romanovs' Poland I was in war with: Latvia, Estonia, Hungary, Romania and Yugoslavia in the same moment, having something like 500, maybe 600k men in the field. And I have won, even if I was in the half way to capitulation, because Romania stretched the frontline so mutch, that they had no enough troops to cover all the borders and I slipped in between the gaps to the VP's. I remember this playtrough as one of the best in my HOI4 carrier.