 The German invasion of the Soviet Union began on 22 June . Over the next two days , the sporadic <unk> by Serbs against the NDH in eastern Herzegovina erupted into mass rebellion , triggered by Ustaše persecution , Serb solidarity with the Russian people , hatred and fear of the NDH authorities , and other factors . Serb rebels , under the leadership of both local Serbs and Montenegrins , attacked police , gendarmerie , Ustaše and Croatian Home Guard forces in the region . In the first few days , the rebels captured gendarmerie posts in several villages , set up roadblocks on the major roads and ambushed several military vehicles . On the night of 26 June , the rebels mounted a sustained attack on the town of Nevesinje in an attempt to capture it , but the garrison held out until the morning of 28 June when NDH troops broke through the rebel roadblocks . 
