I once had the French reemerge while I was at war with the English (they held all of France and England, I was the Danes). They took almost all of England's provinces on the mainland, but none of their provinces on the Isle. Basically they broke the English's back, gave me the oppurtunity to amphibiously assault Britain and if I hadn't been sucked into a war with them could have been quite powerful for some time. Anyway because of their large armies and many rebellions it took me quite some time to put them down.

I'd say they accomplished something, but I suppose you could argue against that.