in july, the germans initiated operation trumpf against the partisans in the southern parts of german-occupied serbia. this was a combined operation with bulgarian troops, serbian quisling formations, and chetnik forces commanded by radoslav račić. račić's fourth group of shock corps from western serbia were reinforced by the rasina-kopaonik group of shock corps commanded by keserović. the total chetnik forces involved in this operation exceeded 10,000 men, and the chetniks were supplied with ammunition and some arms by the germans.