[1] After the end of the First World War (1918), Heusinger returned from British captivity[2] in Yorkshire in December 1919 and in 1920 joined the Reichswehr of the Weimar Republic.

(The Truppenamt functioned as the German Army's covert General Staff; its existence circumvented the 1919 Treaty of Versailles, which had forbidden the institution.)

Heusinger accompanied the field staff and assisted in the planning of operations for the invasions of Poland, Denmark, Norway, and France and the Low Countries.

He reaffirmed that he had not participated in the assassination plot since he still felt an obligation to fulfil his duty as a soldier of the German Reich, despite his personal view that the war had been lost.

He was appointed a Generalleutnant (lieutenant general) on 12 November 1955,[4] in the Bundeswehr and chairman of the Military Leadership Council (Militärischer Führungsrat).

In March 1957, he succeeded Hans Speidel as chief of the all-armed forces department (Chef der Abteilung Gesamtstreitkräfte).

[citation needed] During the war he "bore responsibility for the systematic killings of civilians in Belarus as part of antipartisan operations".

According to documents released by the German Federal Intelligence Service in 2014, Heusinger may have been part of the Schnez-Truppe, a secret army that veterans of the Wehrmacht and Waffen-SS sought to establish in the early 1950s.