After the Japanese attack on Pearl Harbor in December 1941, the 37th Battalion was rapidly raised to help reinforce Fiji and three instructors including Rennie were allowed to join with the rank of sergeant.
After spending two years mainly garrisoning islands and training, Rennie participated in a reconnaissance in force to Nissan Island on 31 January 1944, in which future US Presidents John F. Kennedy and Richard Nixon were also involved [REF].