tagging /u/sovietkangaroo

So, there are several factors coming together in Serbia and in the anti-Partisan and anti-Jewish actions there, that need to be taken into account when answering this question.

For context: The invasion of Yugoslavia was all in all not very well planned and more of an improvised action. It was prompted not because it was part of the long term plan but rather because a.) Mussolini and the Italians had a hard time in subduing the Greek army and were even pushed back into Albania and b.) because of the Nazi leadership's program of securing their flank for their invasion of the Soviet Union.

For the latter, they pressured the Yugoslav government to become part of the Axis, even under terms that the current Yugoslav government had considered favorably, i.e. they weren't obliged to supply troops to the invasion of the USSR. However, a group of Serbian nationalist army officers were very unhappy with Yugoslavia joining the Axis and thus staged a coup d'etat against the government. In the few days that followed, the coup government canceled the Axis pact and tried to ensure their national integrity and sovereignty by concluding treaties of friendship and defense with various neutral and Allied powers but were not very successful. Afraid that their flank was compromised and with the memory of WWI and the Salonika front in mind, the German leadership ordered the invasion of Yugoslavia and Greece. It has also been reported that Hitler was reeling recalling the Serbs of WWI and arguing in line with several typically Austrian prejudices against the Serbs.

While the military invasion itself was quick to be concluded, due to its improvised nature and speedy nature, quite a few members of the Serbian army managed to escape becoming POWs and could retreat with their weapons into the mountains. Also, the imminent attack on the Soviet Union meant in practice that the Germans had very little capacity in terms of occupation troops. This was among the reasons why they agreed so readily to create the NDH in Croatia, grant the Italians and the Bulgarians occupational zones in Greece as well as former Yugoslavia (Montenegro e.g. became Italian occupied as well as swaths of the Adriatic coast), and why it was only Serbia that was occupied. The strange decision however to put the Wehrmacht in charge of Serbia was on the one hand informed by old prejudice of Serbian irregular warfare mixed with the experiences of WWI and on the other hand by the wish to punish Serbia for its perceived "betrayal" regarding the coup d'etat.

This resulted in two important developments: Serbia was occupied but the occupational force consisted of very badly trained and outfitted divisions and in the beginning no serious Serbian collaborationist government was instituted by the Germans. Aside those Serbian army members who were already in the mountains, thousand of Serbian refugee also started to cross the border between Ustasha Croatia and German-occupied Serbia in order to escape the violence in the NDH from late May, early June on and many of those found Chetnik formations willing to take them on to fight against the Ustasha and the Germans.

While the Chetnik formations, with its main strand under the former Serbian army officer Mihailovic had started small acts of sabotage already in May, the situation became a problem for the Germans once the Communists had started their uprising in late June following the German attack on the Soviet Union. Tito and the CPY had the advantage of not only having a number of highly trained and experienced officers coming form their service in the Spanish Civil War but also followed a strategy of not advertising socialist revolution but instead fighting under the banner of national liberation.

Once they join the fight, it is possible to trace to the escalation of the uprising throughout the summer of 1941. On July 12, the Partisans bombed the train tracks at Jrenak, on July 13, they bombed several bridges, on July 16, they cut the telephone line to Belgrade; on July 18, they attempted to assassinate the commander of a German infantry division; in the first ten days of August, the Wehrmacht counted 135 attacks on infrastructure, Serbian police and Wehrmacht troops and that was only the beginning. By the start of September 1941, the Germans had been confined pretty much to the larger urban centers and had ceased control of the country side to the Partisans and Chetniks.

Rather early on, the Germans had responded with shooting hostages as a form of reprisal, including Jews. Starting in July, German troops had by September already shot over 200 Jews and around 500 alleged communists as reprisal for attacks on troops and infrastructure.

During the summer, we are also able to see increased traffic between Belgrade and Berlin, in which the Wehrmacht commander and the plenipotentiary of the foreign office in Serbia are requesting the removal of the male Jews from Serbia. Their logic is simple: Jews are the puppet masters of the communist uprising. Remove the Jews and the Partisan attacks will invariably drop. This had nothing to do with the reality of the uprising. Jews had been early on concentrated in camps and certain areas of Belgrade and while some did indeed join the Partisans, the majority of the rather small Jewish population of Serbia did not have the chance to do so because they were under strict German supervision.