In 1943, with Indonesia now under Japanese Occupation, Umar enlisted with youth groups operating under the supervision of the Japanese Occupational Government. These youth groups provided some physical training which Umar undertook. This was followed in October 1944 by PETA, an auxiliary force consisting of Indonesian recruits which was intended to assist the Japanese in fighting the Allies. When Indonesia declared its Independence, Umar, like many other youths of similar age joined the TKR (The People's Security Soldiers), the forerunner to the TNI.