After his studies, Halem initially joined far-right Black Reichswehr paramilitary troops and became involved in Adolf Hitler's Beer Hall Putsch and the Nazis' march on the Munich Feldherrnhalle on 9 November 1923.

From about 1930, he became active in the conservative Catholic circles around scholar Carl von Jordans in Berlin, whose goal was to keep the Nazi movement from power.

By 1935, under the influence of Ernst Niekisch and Carl von Jordans, Halem already had concluded that killing Hitler was a political necessity to avert a catastrophe.

[1] In 1936 Halem served as an official of the Reich Price Commissioner Carl Friedrich Goerdeler, who soon after fell out with the Nazi government and was replaced by Josef Wagner.

Hitler's plans for the Anschluss annexation of Austria upset Halem, while Ketteler conspired to assassinate the dictator at the time of the German invasion.

Halem was arrested on 26 February 1942 by the Gestapo and suffered torture through a number of prisons and concentration camps, including Sachsenhausen, but did not reveal any of his fellow conspirators.

In June 1944, shortly before the 20 July 1944 coup attempt, the People's Court indicted Halem for conspiracy to commit treason and undermining the war effort.