Probably the HRE, against me as the Danes. I had already taken over the British Isles and beaten the French down. I had been at peace with the HRE, but had gotten excommed when I started on the French. The HRE built a crusade in Lorraine (I think), and sent it towards a lightly defended Denmark. I was pondering how to gather troops to defend the homeland, and then it hit me, "just take Lorraine" (it had been a while since I had played ). Since I had just taken out the French, I had lots of troops in their territory... I jumped Lorraine easily when the crusade move to Freisland, and also did preemptive strikes into a couple of other HRE territories. Within 2 turns, the HRE collapsed as provinces rebelled and I could pick over the remains. I don't know what the HRE king's influence was before this sequence, but I imagine it was pretty much 0 after losing the crusade and a couple of provinces.

Lesson #1: Do not build your crusade in a province next to land owned by the intended target!