Bulgaria once belonged to the Soviet bloc , a group of communist countries.
After WW2 ended, the United States and the Soviet Union entered what we call the Cold War. This involved all of the democratic countries fighting the communist countries. The bulk of the Soviet Bloc was made up of Eastern European nations such as East Germany, Poland, Czechoslovakia, Hungary, Romania, Bulgaria, and Albania. These nations went through many hardships under Soviet rule, and are still suffering today because of it.
I hope this helps! :)