Nazi Germany invaded and occupied Yugoslavia in April of 1941. The Germans used a harsh policy of reprisals in the occupied territories. For every killed German, 100 Yugoslav civilians would be killed, for every injured German, 50 civilians. Britain, fighting in North Africa, Asia, and the Mediterranean can only send a limited amount of equipment to Yugoslav resistance.
The Chetniks were a Serb Nationalist, Monarchist guerilla army that advocated against initiating combat against the Germans and instead collecting arms and supplies for an uprising once the Allies landed in Yugoslavia.
The Partisans were a Communist insurgent group who advocated active and overt resistance against German occupiers"