His Army career began at the age of 18, serving in Flak regiments in Döberitz and Berlin before undergoing police training in 1938, and being assigned to Ordnungspolizei Headquarters.

On 16 April 1945, he was assigned a mission by the Chief of the Army Headquarters (German: Oberkommando des Heeres, OKH) General der Infanterie Hans Krebs.

The task involved attacking a Soviet command post, and Heuer led a group of 28 soldiers to complete the mission.

On the night of 21 April, Heuer's group captured the Russian command post, and seized a quantity of maps and important documents.

Upon his report to Krebs, the general was so pleased with Heuer's successful completion of the mission and his tank destruction efforts that Krebs awarded him the Knight's Cross of the Iron Cross,[Note 1] witnessed by Adolf Hitler's personal adjutant General of the Infantry Wilhelm Burgdorf and SS-Gruppenführer Hermann Fegelein.

[1] As a police investigator in 1952, Heuer attempted to establish the circumstances of the death of his brother-in-law, who had been captured while fighting alongside the 6th SS Mountain Division Nord at Leisenwald and Waldensberg near Wächtersbach.