With the outbreak of the Second World War, the German Army High Command (the OKH) assumed its wartime organization. Heusinger accompanied the field staff and assisted in the planning of operations for the invasions of Poland, Denmark, Norway, and France and the Low Countries. He was promoted to colonel on 1 August 1940, and became chief of the "Operationsabteilung" in October 1940, making him number three in the Army planning hierarchy, after the Chief of the General Staff, General Franz Halder, and the Deputy Chief of the General Staff/Chief Quartermaster, General Friedrich Paulus.