In 1942, von dem Bussche witnessed by chance an SS-organised gruesome massacre of more than 3,000 mostly Jewish civilians carried out by the SD at the old Dubno airport.

Anders Lassen, a Danish resistance hero who fought in the British Army against Germany during World War II, was his first cousin.

On 5 October 1942, at the age of 23, the highly decorated first lieutenant happened to witness the systematic mass execution of over three thousand mostly Jewish civilians—men, women, and children—at the Dubno airfield in Ukraine.

After these events, he asked himself and a small circle of like-minded individuals in the regiment (to which Richard von Weizsäcker, later elected President of Germany, also belonged),[2] why he should still be bound by this oath, which is based on respect and reciprocity, when the Führer had already broken it with the crimes he had ordered.

Three months after his traumatic witnessing of the mass execution, Bussche's decision was clear: it could no longer be a question of sacrificing one's own life on the battlefield, but of using it for Germany against Hitler.

Von dem Bussche, who had been promoted to captain and now worked as a battalion commander in the 9th Grenadier Regiment, was deeply impressed by the encounter with Stauffenberg.

Bussche explained that in view of the crimes he witnessed involuntarily, there were only three ways for an officer to protect his honor "by joining the group of victims" — i.e. to die in battle, to desert, or to rebel against the government that had ordered this and all other massacres.

He later justified his intention to kill with the emergency aid paragraph of the German penal code (§ 32 StGB), which he had had to memorize as a recruit in Potsdam.

In November 1943, von dem Bussche stood by for three days and two nights in the guest barracks at the East Prussian headquarters at Wolfsschanze.

On his arrival he handed co-conspirators Major i. G. Joachim Kuhn and Colonel Helmuth Stieff the documents given to him by von Stauffenberg for the implementation of the coup d'état.

After the failure of the assassination plan, Major Kuhn buried the documents together with the explosives on the premises of the Oberkommando des Heeres (OKH).

These were later found by Soviet officers on 17 February 1945, based on Kuhn's description of the hiding place, and in 1997 a copy was handed to German Chancellor Helmut Kohl by Russian President Boris Yeltsin.

As a result of his injuries, von dem Bussche spent several months in the Waffen-SS hospital Hohenlychen in Lychen (a privilege allotted to him as recipient of the gold German Cross).

This hospitalization helped von dem Bussche escape discovery during the wave of persecution that followed the 20 July assassination attempt and he was never betrayed by any of the officers who knew of his involvement.

The von dem Bussche family owned manors in Thale, the former Wendhusen monastery, and Stecklenberg, however immediately after the Second World War, these properties were seized by the Soviet occupiers of what was to become communist East Germany.

From 1959 to 1962 he was head of the Salem Castle boarding school founded by Kurt Hahn, Karl Reinhardt and the Margrave of Baden.

In 1991 he was one of many unsuccessful plaintiffs who sued the federal government for return of the properties that had been expropriated in 1946 by the Soviet occupation forces.