At the Headquarters of Army Group Centre on the Eastern Front, the First General Staff Officer, Oberstleutnant Henning von Tresckow, had collected around himself a staff of like-minded officers - men fundamentally opposed to the criminal Nazi regime. Even before the invasion of the Soviet Union in June 1941, Tresckow had been shocked and appalled by the criminal nature of the orders issued to the subordinate commands. He recognised that such orders as the "Commissar Order" and the "Barbarossa Instructions" were clear violations of international law, and he convinced his commander, Feldmarschall von Bock to protest to the C-in-C of the Army, Generaloberst von Brauchitsch – to no effect. By the winter of 1941, he and the men around him at Army Group Centre Headquarters had seen exactly where these barbaric orders led: to atrocities against the helpless and unarmed, whether prisoners of wars or Russian civilians. After becoming witnesses to a large-scale massacre of Jews, Tresckow decided that Hitler and his regime could be tolerated no longer. Hitler had to be eliminated – like a mad dog.