 Complete the following phrase:  Until the middle of June 1944, the 1st Proletarian Corps operated in Western and Central Bosnia and Lika, where it fought heavy battles with units of the German 15th Mountain, 5th SS Mountain and 69th Corps. In 1943-1944, it participated in the Banja Luka Operation and Drvar Operation. In May 1944 it withstood the German 