Nowadays, if you hear “Poland” or “Lithuania,” (and you know something of geography) you know they are two separate countries… but this was not always so!
Beginning in the 14th century, all the way to the 18th, Poland and Lithuania were a dual-nation, ruled by one King (and the parliament called the “Sejm.”) The country was absolutely HUGE in comparison to contemporary Poland and Lithuania, and was the most powerful and populous country in Europe for a couple of centuries. No wonder Russia, Prussia, and Austria had to gang up to break it up!
The first few hundred years of the union were ruled under the Jagiellonian Dynasty (who were of Lithuanian ancestry, and married into the Polish aristocracy.) King Władysław Jagiełło was responsible for the defeat of the Teutonic Knights at the Battle of Grunwald in 1410, which was the beginning of the downfall of the German religious state.