The Polish-Lithuanian Commonwealth

 Poland and Lithuania have an interesting history together, they were once the biggest European nation in it's prime, until corruption overtook the nation and signaled its downfall and fell to the surrounding empires.Poland and Lithuania were extensive nations in their prime. Poland Stretched from the city of Gdansk all the way to Moldova. Lithuania, the bigger of the two grew from a giant power pool left by the Mongolians, this allowed Lithuania to become gigantic, even nearing the capital of Russia, Moscow.

Poland like every country during that time was a monarchy, Lithuania who was close allies to Poland eventually fell under a personal union. the two countries would soon unite after 80 or so years and it seemed like it would survive a long time, as it was pushing back Russia and taking lands it lost back from them. However their government was what doomed the empire. The Polish-Lithuanian Commonwealth had a Elective Monarchy, basically the government voted the king, you can sorta see the problem. The government got corrupt around the 1780's and they had to choose between to people and they honestly chose the terrible one, as they got richer because of it, this lead to the nation being very weak and soon Russia, Prussia and Austria annexed land from them. The Polish-Lithuanian Commonwealth was no more. In the modern world Poland and Lithuania are still allies but no one is sure if they still want what they were before.