Before World War I started, there were two alliances in Europe. The Triple Alliance included Germany, Austria-Hungary and Italy. The Triple Entente included Russia, France, and Great Britain. The alliance system was one reason why World War I began. Once World War I began, the alliances were called the Central Powers and the Allied Powers.
Once World War I started, there was some shifting of alliance members. Italy switched sides and joined the Allies. Italy was unhappy with what their alliance members offered them if they won the war. A few years into the war, the United States joined the Allies. The Central Powers also reviewed additional support. The Eastern Europe country of Bulgaria joined with the Central Powers during World War I. Unfortunately for Bulgaria, they were on the losing side in World War I. They had to give up some land when the war ended.