The nation of Yugoslavia was created in the aftermath of World War I, and it was mostly composed of South Slavic Christians, though the nation also had a substantial Muslim minority. Clear ethnic conflict between the Yugoslav peoples only became prominent in the 20th century, beginning with tensions over the constitution of the Kingdom of Serbs, Croats, and Slovenes in the early 1920s and escalating into violence between Serbs and Croats in the late 1920s after the assassination of Croatian politician Stjepan Radić. This nation lasted from 1918 to 1941, when it was invaded by the Axis powers during World War II, which provided support to the Croatian fascist Ustaše (founded in 1929), whose regime carried out the genocide of Serbs, the Jews and of the Roma inside its territory through executions in concentration camps and other systematic mass crimes.