He joined the Army of the German Republic (Reichsheer); between 1921 and 1933, he held several staff and command positions, and then served in the Ministry of Defense.
On 1 April 1933, after Hitler came to power, Marcks was transferred to the army, serving as Chief of Staff of VIII Corps.
In the summer of 1940, Franz Halder, chief of OKH General Staff, directed Marcks to draft an initial operational plan for the invasion of the Soviet Union.
Citing the need to "protect Germany against enemy bombers", the report advocated the A-A line as the operational objective of the invasion of "Russia" (sic).
Marcks envisioned that the campaign, including the capture of Moscow and beyond, would require between nine and seventeen weeks to complete.