Yugoslavia

In 1918, after World War I, the Slovenes joined with other southern Slav peoples in forming the State of Slovenes, Croats and Serbs (October 29, 1918) and then the Kingdom of Serbs, Croats and Slovenes (December 1, 1918) under King Peter I of Serbia. Renamed in 1929, the Kingdom of Yugoslavia fell to the Axis powers during World War II, when Germany, Italy and Hungary each annexed parts of Slovenia, the largest part being the Südsteiermark annexed to the "Ostmark" (Nazi German Austria).

Following Yugoslav partisan resistance to German, Hungarian, and Italian occupation and elimination of rival resistance groups that were forced into open collaboration with Italian and/or German forces while fighting communism, Josip Broz Tito established the Socialist Federal Republic of Yugoslavia in 1945, of which Slovenia formed a constituent republic.

Slovenia continued to form Yugoslavia's most prosperous and advanced republic throughout the communist era, at the forefront of Yugoslavia's unique version of communism.