Germans began searching for a Serb suitable to lead a puppet government in Belgrade. They initially settled on Milan Aćimović, a staunch anti-communist who served as Yugoslavia's Minister of Internal Affairs in late 1939 and early 1940. Occupation and resistance Two resistance movements emerged following the invasion: the communist-led, multi-ethnic Partisans, and the royalist, Serbian nationalist Chetniks, although during 1941, within the occupied territory, even the Partisans consisted almost entirely of Serbs. The Partisans were led by the General Secretary of the Communist Party of Yugoslavia, Josip Broz Tito; the Chetniks were led by Colonel Draža Mihailović, an officer in the interwar Royal Yugoslav Army. The two movements had widely divergent goals. Whereas the Partisans sought to turn Yugoslavia into a communist state under Tito's leadership, the Chetniks sought a return to the pre-war , whereby the Yugoslav monarchy—and, by extension, Serb political hegemony—would be restored. Communist resistance commenced in early July, shortly after the invasion of the Soviet Union, targeting both the Germans and the puppet authorities. By late August 1941, the Partisans and Chetniks were carrying out joint attacks against the Germans. The Partisans were well organised and many of their commanders had ample military experience, having fought in the Spanish Civil War. Within several months of the invasion, they had 8,000 fighters spread across 21 detachments in Serbia alone. Many Chetniks were either veterans of the Balkan Wars and World War I or former members of the Royal Yugoslav Army. They boasted around 20,000 fighters in the German-occupied territory of Serbia at the time of the massacre. On 29 August, the Germans replaced Aćimović with another fervent anti-communist, the former Minister of the Army and Navy and Chief of the General Staff, General Milan Nedić, who formed a new puppet government. In September, the Nedić government was permitted to form the Serbian Volunteer Command (; SDK), an auxiliary paramilitary formation to help quell anti-German resistance. In effect, the SDK was the military arm of the fascist Yugoslav National Movement (, Zbor), led by Dimitrije Ljotić. It was originally intended to have a strength of 3,000–4,000 troops, but this number eventually rose to 12,000. It was headed by Kosta Mušicki, a former colonel in the Royal Yugoslav Army, whom Nedić appointed on 6 October 1941. In the early stages of the occupation, the SDK formed the bulk of Nedić's forces, which numbered around 20,000 men by late 1941.