After the war he ran a publishing house which specialised in military literature and instructional manuals.

Shortly before the Second World War began in 1939, Zimmermann was reintegrated in the new armed forces of Nazi Germany, the Wehrmacht.

In October 1940, he was appointed Generalstabsoffizier at the headquarters of Army Group D, under the command of Erwin von Witzleben.

Zimmermann was promoted to the rank of Oberstleutnant (lieutenant colonel) on 1 August 1941, under the command of Gerd von Rundstedt.

On 5 June (the day before D-Day) Zimmerman the Operations Officer at OB West in France received a message from Hans Speidel at Rommel’s headquarters that the Fifteenth Army was on alert.