After the defeat of the Austro-Hungarian empire, the
State of Slovenes, Croats and Serbs declared independence on 1 October 1918, however on 1 December 1918,
Regent Alexander announced the union of the
Kingdom of Serbia with the State of Slovenes, Croats and Serbs to form the
Kingdom of Serbs, Croats and Slovenes. On 5 December 1918, Metzger led a minor revolt of Croat soldiers in
Zagreb, contemporaneously with other regional uprisings, Metzger was arrested several times, including for his participation in the so-called () of 1919. Acquitted on 7 April 1920, Metzger worked for the Hungarian Defense Ministry. During 1930, Metzger, then a Hungarian intelligence officer, engaged with other members of the
Party of Rights in organisation of proto-Ustaše activity among Croats in towns along the border of the
Kingdom of Yugoslavia. Metzger's German descent was a rarity among the small number of pre-World War II Ustaše members, given the Ustaše's racialist principals. He was allegedly one of the organisers of the 1934 assassination in Marseilles of King
Alexander I of Yugoslavia. ==World War II==