He also fought with Serb forces during World War I, attaining the rank of Chetnik commander (Serbo-Croatian: vojvoda, војвода)[1] and losing an arm in combat.

[1] An organisation with considerable influence with the Serbian public, Narodna Odbrana petitioned Prince Paul on various occasions urging him to resist pressure from Adolf Hitler for Yugoslavia to join the Tripartite Pact.

Trifunović-Birčanin was in close contact at this time with the British Special Operations Executive (SOE), which was actively attempting to prevent Yugoslavia from joining the Axis powers.

[7] The coup by predominantly Serbian military officers led by the Head of the Air Force General Dušan Simović took place on 27 March, and Prince Paul was replaced by King Peter II.

One such Chetnik-Italian agreement was concluded at a meeting in Split on 20 October 1941 by Trifunović-Birčanin, Dobroslav Jevđević, a leading Chetnik in the inter-war kingdom, and Angelo de Matteis, head of the information division of the Italian 6th Army Corps.

[11] In addition to Jevđević, with whom he worked closely on liaison with the Italian forces, Trifunović-Birčanin's subordinate commanders included Momčilo Đujić (northern Dalmatia), Ilija Mihić and Slavko Bjelajac (Lika), and Petar Baćović (Herzegovina and southeastern Bosnia).

[9] In early January 1942, Trifunović-Birčanin played a central role in organizing the units of Chetnik leaders in western Bosnia, Lika, and northern Dalmatia into the Dinara Division and dispatched former Royal Yugoslav Army officer to help.

"[12] In the same month General Renzo Dalmazzo, Italian Sixth Army Corps commander organised a meeting in the hope that the Chetniks would take part in a joint operation against the Partisans.

On 10 and 21 September, Trifunović-Birčanin met with Mario Roatta, commander of the Italian Second Army, and urged him to take action "as soon as possible" in a large operation against the Yugoslav Partisans in the Prozor-Livno area and offered aid in the form of 7,500 Chetniks on the condition that they be provided the necessary arms and supplies.