With the collapse of the Habsburg Empire after World War I, political representatives of Croats and Serbs in Croatia voted to join with the kingdom of Serbia in a new Kingdom of Serbs, Croats, and Slovenes (Yugoslavia). But the authoritarian rule of the Belgrade-based governments and of the Serbian king caused resentment in Croatia. In a compromise meant to stabilize the country in the face of imminent war in the rest of Europe, Belgrade in 1939 granted Croatia limited autonomy. But the events of World War II soon overwhelmed Yugoslavia, and in April 1941 the advancing German army declared an independent Croatian state, ruled by a small fascist group called the Ustaša. This group ruled through terror, targeting Serbs for extermination or forced conversion to Roman Catholicism.
After the war, Croatia was included as one of the six republics of Marshal Tito's communist-ruled Yugoslavia. Although manifestations of national sentiment were suppressed, the 1960s saw an upsurge of Croatian national feelings, triggered by economic reforms, which sought to decentralize the country and thus cede more power to local officials. By 1971, a full-scale Croatian Spring was underway, in which support for Croatian nationalism was a means of expressing dissatisfaction with the existing system. The ruling League of Communists of Croatia (LCC) co-opted the nationalist issue in an attempt to gain legitimacy and to keep the nationalist wave from getting out of control. In 1971 and 1972, however, Tito cracked down on these Croat reformists, purging them and replacing them with more conservative officials, but also giving Croatia and other Yugoslav republics a greater degree of autonomy.
After Tito's death in 1980, political and economic difficulties escalated. The federal government began to fall apart as attempted reforms failed. The conservatives in the Serbian leadership sought to quash reemerging reformist trends and to reimpose a greater degree of centralization of the ruling party and state, in particular by trying to subvert the leadership of the other Yugoslav republics. The result was that by 1989, the previously conservative Croatian party, reacting to Serbian pressure, came under the control of more liberal forces that in December of that year scheduled the republic's first multiparty elections for April 1990. The Croatian Democratic Union (HDZ) won those first free postwar elections on a platform of nationalism, anticommunism, and privatization. The HDZ was comprised of Croat nationalists ranging from far right émigrés with connections to the wartime Ustaša regime to moderate, democratically-oriented officials who had been thrown out of the Communist Party in the purges of the early 1970s. In the presidential election, HDZ candidate Franjo Tudjman won with 55% of the vote. He remained in office until his death in 1999.
On 25 June 1991, Croatia declared its independence. One month later, the conflict between Serbs and Croats in Croatia escalated into civil war begun by Serbian invasion. The United Nations (UN) initiated a ceasefire in 1992, but the agreement only stayed in effect for one year, when Croatia fought to regain territory taken by Serbs. Civil wars continued, despite repeated UN attempts for ceasefire agreements, until August 1994, when Croatian forces recaptured Krajina with a major offensive, and some 150,000 Serbs fled the region, many to Serb-held areas in Bosnia and Herzegovina.