After Balkan wars a part of Croats began to envisage Serbia to be to the other South Slavs what Piedmont was to other Italians : a unifying force that will help create an independent South-Slavic state . In World War I , Croats fought in the Austro-Hungarian army against Serbia , while Croatian general Ivan Salis-Seewis was a military governor of occupied Serbia . Some Croat POWs volunteered to fight in Thessaloniki battlefront with Serbian army . On 29 October 1918 the Croatian Sabor declared independence from Austria-Hungary and decided to join the newly formed State of Slovenes , Croats and Serbs which on 1 December 1918 entered into union with the Kingdom of Serbia and formed the Kingdom of Serbs , Croats and Slovenes . Initial Croatian zeal for the new state faded away as the republican view of a new state was ignored and especially since the concept of " Greater Serbia " was put in practice during the early 1920s , under the Yugoslav premiership of Nikola Pašić . Using tactics of police intimidation and vote rigging , he diminished the role of the oppositions ( mainly those loyal to his Croatian rival , Stjepan Radić ) to his government in parliament , creating an environment for centralization of power in the hands of the Serbs in general and Serbian politicians in particular . Police violence further alienated Croats , who began to ask for their own state . On 20 June 1928 Stjepan Radić and five other Croat politicians ( supported by a vast majority of Croats ) were shot in the national assembly in Belgrade by a Serb deputy , enraged by continuous Croatian claims that they were ' exploited by Serbia and that Serbia is treating them like a colony ' . This led to the royal dictatorship of King Alexander in January 1929 . The dictatorship formally ended in 1931 when the king imposed a more unitarian constitution and changed the name of the country to Yugoslavia . The HSS , now led by Vladko Maček , continued to advocate federalization of Yugoslavia , resulting in the Cvetković – Maček Agreement of August 1939 and the autonomous Banovina of Croatia . In April 1941 , Yugoslavia was occupied by Germany and Italy who created a puppet-state called the Independent State of Croatia ( NDH ) which was governed by the pro-Axis Ustaša movement , which sought to ethnically cleanse Serbs , Jews , Roma , and many others from its territory thus committing a genocide . The Simon Wiesenthal Center estimates that at least 100,000 Serbs , Jews , Roma , anti-fascist Croats , and others were murdered in Jasenovac concentration camp alone . Initially , the Independent State of Croatia was supported by Vladko Maček and a vast majority of Croats who had welcomed German occupying forces in Zagreb as liberators during the Axis invasion of Yugoslavia . Following the victory of Yugoslav Partisans , who were led by the Croatian communist Josip Broz Tito , the Ustaša and the Chetniks were defeated . Yugoslav communists abolished the monarchy and established one-party socialist republic and a federation governed by the League of Communists of Yugoslavia . Newly formed socialist Yugoslav state under Tito 's benevolent dictatorship was made up of six socialist republics including SR Serbia and SR Croatia
committing a genocide
Persecution of Serbs in the Independent State of Croatia
Croatia–Serbia relations