Halder began his military service in 1914. In 1937 he met and became a loyal supporter of Adolf Hitler. Halder participated in the strategic planning for the 1939 German invasion of Poland. The plans authorised the SS to carry out security tasks - on behalf of the army - that included the imprisonment or execution of Poles. In July 1940 he began planning for the Axis invasion of the Soviet Union, Operation Barbarossa, which began on 22 June 1941. That summer Halder engaged in a long-running and divisive dispute with Hitler over strategy. Hitler removed Halder from command in September 1942. After the 20 July 1944 plot to assassinate Hitler, Halder was arrested as it came to light that he had been involved in an earlier plot, leading to his imprisonment. As chief of OKH General Staff, he had kept extensive notes, later published as "The Halder Diaries".