This reminds me of a MTW(1) campaign I played a couple of years ago. I was playing as Denmark as I liked the forced slow start it gave you. I'd built up to about four or five provinces and I don't believe I could have expanded much quicker. To the South was the HRE but I could only see the provinces I bordered as I had not developed much of a spy or ship network at that point.

All of a sudden, Five stacks of Spanish troops conquered a HRE province next to me. Stunned by their sudden appearance I removed the fog of war and found that they managed to blitz all of the Spanish Peninsula, North Africa and Western Europe. They were already pushing into England and the five stacks on my border were the tip of the iceberg of what I was about to face.

I quit the campaign as I didn't have a hope of winning against such a juggernaut but I thought it was fantastic that the AI was capable under the right conditions of stealing such a victory. There was nothing I could have done to prevent there build up although if I had built up quicker it may have turned into some sort of clash of the titans.

Now, every time I build a mighty empire and have to spend the entire late game mopping up all the little ones that are left, I find myself thinking of that day.