Yugoslavia used to be a large country in Southeastern Europe. It originally formed after World War 1, and gained international recognition in 1922. During World War 2, Yugoslavia was invaded by the Axis powers, and was subsequently split up, with various parts being occupied by Germany, Italy, Hungary, and Bulgaria. Yugoslav nationalists waged a guerilla campaign to drive the Axis powers out, and had a lot of success, finally expelling the Axis powers in 1945.
Following World War 2, Yugoslavia reorganized as a communist state, initially modeling itself after the Soviet Union but quickly deciding to go a different way and forge their own path to socialism. This worked for a while, but eventually ethnic tensions between the demographically dominant Serbs and the rest of Yugoslavia’s citizens caused parts of it to start breaking away from the union. Once the process was started, it all fell apart fairly quickly going from being fully united in 1989 to fully dissolved in 1992.