The Hanseatic Wars

Belligerents:
Outcome: The crowning of a French king as Holy Roman Empire; disintegration of any significant imperial authority.

One of the shortcuts that alternate-history writers take is using their plots to kill things that they're too lazy to research. While I'm endeavoring to not do this to ethnicities overly much, I'm not going to lie: I do it too. (And it's problematic for me because I have enough gaps in my knowledge of how things work that when I try to do this I still need to do all the research anyways.)

This is a related thing: using the plot to cleanly end a few crises ahead of schedule.

The wars commonly called "Hanseatic" (what they're called depends on which language and locale you're speaking in, because the politics of the matter are rather obscure) are the substitute for the Thirty Years' War: a crisis that plunges Germania into chaos. Unlike the Thirty Years' War, there are different demographic issues at work here.

And by the end of it, both the Hanse and the Reich have been crippled thoroughly enough to render them moribund.