Through the ensuing Japanese Malayan campaign during the World War II, Leong is the leader of the Malayan Kuomintang-guerrilla branch of the Overseas Chinese Anti-Japanese Army (OCAJA) in Perak where the group controlled the upper portion of the Perak River Valley. After the war ended and subsequent Japanese surrender, a negotiation was held between him and the British Malayan authorities where he agreed for the OCAJA under his authority being absorbed into the national Special Constabulary in 1947 to fight alongside the British against the Communist-influenced Malayan National Liberation Army (MNLA).