He settled in Belgrade where he remained an active member of the Russian All-Military Union and a professor at the Higher Military Courses of Belgrade.
During World War II as an ardent anti-communist he sided with the Axis powers and the Russian Liberation Movement, and briefly served in the propaganda services of the Wehrmacht as head of the Russian section.
In early 1945 he returned to active front-line service as a Major in Boris Smyslovsky's 1st Russian National Army.
Through Liechtenstein Messner and his family moved in 1947 to Argentina, where he continued his earlier work as a journalist, publisher, writer and military theorist.
His most well-known publication, "Mutiny, or the name of the Third World War" (Russian: ????? — ??? ??????? ?????????), predicted that the future wars would be waged by small terrorist cells and special forces, gaining influence by subversion and organised revolutions rather than through traditional warfare.
Messner died in Buenos Aires in 1974 and was buried at the British Cemetery there.