 Complete the following phrase:  During the first months after Operation Barbarossa, the 22 June 1941 German invasion of the Soviet Union, Grechkin was tasked with organizing the construction of the Mius defensive line, which protected Rostov-on-Don. From 3 August to 4 September, he was the acting commander of the North Caucasus Military District. Grechkin became commander of the 56th Army operational group in October, leading it in the 