Adding to the list of countries that sort of get along, but sort of don't at the same time.
TW: mentions of suicide and substance abuse.
Austria-Hungary and the German Empire.
Now, these two weren't fighting much in terms of wars against each other, but that didn't mean they were buddy-buddy. (It's family issues) It's hard to sweep centuries of loathing under the rug, particularly when you're brought up to challenge each other. AH's sister had to die so that the German Empire could rise after all. He certainly hated how GE lost nothing in the war, whereas he lost almost everyone. He originally tried to ally with Italy against Prussia and the North German Confederation, but this fell through. But as was the usual case with alliances, there was something else to seal the deal. The dual Alliance began a very unhappy marriage. German-Austria, was the first born, a short lived republic. GE was not very attracted to her husband and found him not very witty either. She had a short affair with a cityhuman, in which her son, the Wiemar Republic, was born. AH was not very pleased that his wife had left his empire glory for a city. They had a second child, TR, as a last ditch effort to save their marriage. Needless to say, it didn't work.
WW1 was not very fun. AH declares war, gets declared war on, GE decides to join in, with minor begging on the part of AH. But as the war drags on, he depends more on her support. They both drink a bit too much, GE dabbles in drugs, and the war takes a turn for the worse.
AH talks about defeat, GE gets very mad about that, tells him that he'll burn in hell if he leaves her hanging, and both of them begin to fracture. AH begins ignoring calls, and then a teenage Austria (Who GE hates because she believes is a child of another woman) shows up on her doorstep with the news that his father has committed suicide. GE throws a rage, Prussia slowly takes the three kids along with Austria, to his niece's estate nearby, war ends, GE's executed.