Yugoslavism is a Slavic nationalist political ideology that has historically advocated the union of all South Slavic countries, namely Bosnia and Herzegovina, Bulgaria, Croatia, Montenegro, Serbia, Kosovo, Slovenia, and Macedonia. The concept first developed in the 1830s when Croatian writers began to advocate a South Slavic renaissance as a precursor to the unification of all of the South Slavic nations, and the movement became popular in the years both before and during World War I. After the war's end, the Kingdom of Yugoslavia was formed out of the original Kingdom of Serbia and the Kingdom of Montenegro and the South Slavic lands once controlled by Austria-Hungary, and a strong sense of Slavic nationalism developed during the period of German and Italian occupation during World War II. Yugoslavism was strong under Josip Broz Tito and his League of Communists of Yugoslavia, but it disintegrated after Tito's death as sectarian tensions grew between the ruling Serbs and the under-represented Bosniaks, Croatians, Albanians, and Slovenes. After the breakup of Yugoslavia during the 1990s, Yugoslavism remained a sizeable movement as a part of the "Yugo-nostalgia" phenomenon, as many people recalled how life was better under a single country.