In April 1941, Yugoslavia was occupied by Germany and Italy. Following the invasion the territory of Slavonia was incorporated into the Independent State of Croatia, a Nazi-backed puppet state and assigned as a zone under German occupation for the duration of World War II. The regime introduced anti-semitic laws and conducted a campaign of ethnic cleansing and genocide against Serb and Roma populations, exemplified by the Jasenovac and Stara Gradiška concentration camps, but to a much lesser extent in Slavonia than in other regions, due to strategic interests of the Axis in keeping peace in the area. The largest massacre occurred in 1942 in Voćin.