First Yugoslavia (1918–1941)

Shortly before the end of the Great War in 1918, the Croatian Parliament severed relations with Austria-Hungary as the Allied armies defeated those of the Habsburgs. The People's Council (Narodno vijece) of the state, guided by what was by that time a half a century long tradition of pan-Slavism, joined Serbia and Montenegro in the Kingdom of the Serbs, Croats and Slovenes shortly thereafter.

The Kingdom underwent a crucial change in 1921, when the new constitution centralized authority in the capital of Belgrade and redrew internal borders to favor the Serb majority, to the dismay of the Croatians led by the Peasant Party of Stjepan Radic. They boycotted the government of the Serbian Radical People's Party throughout the period, except for a brief interlude between 1925 and 1927.

In 1928, Radic was mortally wounded by a Serb deputy during a Parliament session which caused further upsets in Zagreb. In 1929, King Aleksandar proclaimed a dictatorship and imposed a new constitution which, among other things, renamed the country Kingdom of Yugoslavia.

In 1934, the king Aleksandar was assassinated abroad, in Marseilles, by a coalition of two radical groups: the Croatian Ustase and the Macedonian VMRO. Croatia received some autonomy in 1939 with a reshuffling of the provinces, but the militarist regime in Belgrade crumbled in 1941 and the Axis powers quickly occupied Yugoslavia.

World War II (1941–1945)

The Axis occupation of Yugoslavia in 1941 allowed the Croatian radical right Ustase party to come into power, forming the so-called "Independent State of Croatia", led by Ante Pavelic, he was styled (Fuhrer-like) Poglavnik Nezavisne Drzave Hrvatske (i.e. Leader of the Independent State of Croatia). His fascistoid puppet regime enacted racial laws, formed eight concentration camps and started a campaign to exterminate Croatia's ethnic minorities (Serbs, Romas and Jews in presice) and remove the "enemies of the state".

The anti-fascist partisan movement emerged early in 1941, under the command of the Communist party, led by Josip Broz Tito, as in other parts of Yugoslavia. Serbian royalist guerilla Cetnici were also formed.

Early in the war, Ustase opened up the Jasenovac concentration camp. This complex of internment and extermination camps was one of the larger sites of mass murder in occupied Europe at the time and was the place of death of tens of thousands of people.

Both Ustase and Cetnici collaborated with the Axis powers and fought together against the Partisans. By 1943, the partisan resistance movement greatly expanded and was able to expel all Nazi collaborators by 1945, with the help of the Soviet Red Army. The ZAVNOH, state anti-fascist council of people's liberation of Croatia, functioned since 1943 and formed an interim civil government.

By the end of the war, hundreds of thousands of Serbs, Jews and Romas, as well as communist-orientated people were executed by the regime. The presice figure is unknown; as the Ustase destroyed most of their intel before capture; although 400,000 is generally the most accepted figure. After the war, Between 50,000 and 200,000 of Croatia's population were massacred in places like Bleiburg