Busch remained in the army and in 1925 was appointed Inspector of Transport Troops and in 1930 was promoted to the rank of lieutenant colonel and placed in command of the 9th Infantry Regiment.
An ardent supporter of the Nazi Party, Busch achieved rapid promotion after Adolf Hitler came to power in 1933. Within two years he had been promoted to major general and was commander of the 23rd Infantry Division. In February 1938, Hitler appointed him general. Whereas most senior officers pleaded with Hitler to move with caution, Busch and fellow Nazi, Walther von Reichenau, urged him to invade Czechoslovakia.
Busch served under Siegmund List during the invasion of Poland in September 1939. The following year he led the 16th Army during the Western Offensive. On the May 26, 1940, Hitler awarded him the Knight's Cross.
Busch took part in Operation Barbarossa and on September 8, 1941, his 16th Army took Demyansk before taking part in the siege of Leningrad. Despite a counter-attack by the Red Army Busch's troops held the line from Staraya to Ostashkov. After a brave defence of his position he was promoted to field marshal.
In October 1943, Busch replaced General Gunther von Kluge as head of Army Group Centre. However, he only held the post until June 1944 when he was replaced by General Walther Model.
Busch was recalled in March 1945 when he became head of Army Group Northwest. Along with Kurt Student and his 1st Parachute Army, Busch had the task of trying to halt the advance of General Bernard Montgomery and the Allies into Germany.
Ernst Busch surrendered on May 3, 1945. He was taken to Britain as a prisoner of war and he died in captivity on July 17, 1945.