Brunner served as head of the Einsatzkommando 4/I during the invasion of Poland and the early stages of the German occupation in 1939, tasked with the killing of Polish civilians.

During his time in Northern Italy he was also responsible for the arrest, and ultimately, the deportation of the Jews in his area of jurisdiction, as well as reprisals against Italian civilians.

During World War I, from September 1917 onward, he served in the Bavarian 16th Infantry Regiment "Großherzog Ferdinand von Toskana", from which he was discharged in 1919.

[1] Leaving the Bavarian Army with the rank of lieutenant, Brunner joined the Freikorps, a right-wing paramilitary militia, in 1919 and was part of the Marine-Brigade Ehrhardt in 1922–23.

[1] In this role he was responsible for securing employment in an aircraft factory for Max Troll, a communist-turned-informer who betrayed over 250 resistance members to the Gestapo between 1933 and 1936.

[4] With the invasion of Poland, Brunner served as head of the Einsatzkommando 4/I until November 1939,[5] tasked with the killing of Polish civilians as part of Operation Tannenberg.