The invasion of June 1941 was the largest operation undertaken by German forces during the war, comprising more than 100 army divisions and some 17 Panzer divisions along with 3,000 aircraft. Facing these forces were more than 150 Russian divisions along with 54 tank brigades. Despite the size of the Russian forces, which were expanded within six months by a further 300 divisions, the German and Axis forces swept eastwards at an unprecedented rate, capturing more than 3.3 million prisoners by December 1941.