ow 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. The Yugoslav government retained control of defence, internal security, foreign affairs, trade, and transport while other matters were left to the Croatian Sabor and a crown-appointed Ban.
In April 1941, Yugoslavia was occupied by Germany and Italy. Following the invasion the territory, parts of Croatia, Bosnia and Herzegovina, and the region of Syrmia were incorporated into the Independent State of Croatia (NDH), a Nazi-backed puppet state. Parts of Dalmatia were annexed by Italy and the northern Croatian regions of Baranja and Međimurje were annexed by Hungary. The NDH regime was led by Ante Pavelić and ultranationalist Ustaše. The regime introduced anti-semitic laws and conducted a campaign of ethnic cleansing and genocide against Serb and Roma inhabitants of the NDH, exemplified by the Jasenovac and Stara Gradiška concentration camps. It is estimated that out of 39,000 Jews in the country only 9,000 survived; the rest were either killed or deported to Germany, both by the local authorities and the German Army itself. Croatian and Serbian sources disagree on the exact figures. Furthermore a total of around 320,000 to 334,000 Serbs were estimated to have been killed in the territory of present-day Croatia and Bosnia-Herzegovina—roughly corresponding to NDH territory, of all causes - either by the regime, as members of armed resistance, or as Axis collaborators. In total the estimated number of Serb casualties throughout Yugoslavia in the war was around 537,000. At the same time, around 200,000 Croats were estimated to have been killed during World War II, likewise in various roles.
A resistance movement soon emerged. On 22 June 1941 the 1st Sisak Partisan Detachment was formed near Sisak, as the first military unit formed by a resistance movement in occupied Europe
