He was promoted to Leutnant (second lieutenant) on 1 April 1922 and to Oberleutnant (first lieutenant) on 1 February 1927. He became adjutant of the III. (Jäger) Bataillon (3rd light infantry battalion) of the 17. Infanterie Regiment (17th infantry regiment) on 1 October 1927. In 1932 he served with the 12. (MG) Kompanie (12th machine gun company) of the regiment. Transferred to the Kriegsakademie (war academy) in the fall of 1932 he was promoted to Hauptmann on 1 April 1934. The treaty of Versailles had imposed severe restrictions on Germany's military strength. The Weimar Republic largely obeyed the Versailles restrictions, but with Adolf Hitler's rise to power the remilitarisation began. The Reichswehr, renamed the Wehrmacht, expanded. Foertsch at this time served in the military headquarters in Königsberg. He was appointed company chief in the Infanterie-Regiment 81 on 12 October 1937 and was promoted to Major on 1 August 1938. He was transferred to the Generalstab (General Staff) of the III. Armeekorps (3rd Army Corps) on 10 November 1938, a position he held at the outbreak of World War II. In the fall of 1939 he became the first officer of the general staff (Ia) of the 60. Infanterie-Division (60th infantry division) and participated in the Battle of France. After the invasion of France he was relieved of his position and was transferred to the chief of staff of the Heeresrüstung and Ersatzheer (Army Equipment and Replacement Army) and attained the rank of Oberstleutnant on 1 November 1940. On 1 June 1942 he was promoted to Oberst im Generalstab and at the same time appointed first officer of the general staff of the 18th Army. Foertsch earned the German Cross in Gold on 10 May 1943 for his support in the heavy fighting in the northern sector of the eastern front. He was appointed chief of the general staff of the 18th Army on 1 December 1943 and promoted to Generalmajor on 1 June 1944. Three months later he received the Knight's Cross of the Iron Cross. He was appointed chief of the general staff of the Heeresgruppe Kurland at the end of January 1945. His promotion to Generalleutnant followed on 1 March 1945. After the capitulation of all German forces he was taken prisoner of war by the Red Army of the Soviet Union. He was sentenced to 25 years' imprisonment for war crime charges against the destruction of cities