Immediately after the German invasion of Poland, Hamann was given the command of a frontier defense zone on the Polish border, which he held until 8 January 1940.

[1] On 1 June 1942, he was promoted to major general and sent to serve as the military commander of Oryol, in the German-occupied area of the Soviet Union.

During the Battle of Kursk, while still commandant of Oryol, he was the chief of Gruppe Hamann - a support formation which consisted mainly of the 3rd Brandenburg Regiment and existed from 20 July to 1 August as part of General Lothar Rendulic's XXXV Corps.

[3] On 17 July, he was paraded through the streets of Moscow with 50,000 other captured German soldiers, in the aftermath of Bagration.

[4] On 30 December 1945, a Soviet military tribunal convicted him of war crimes against the civilian populations of Bryansk[5][6] and Bobruisk.