Rennie later wrote in his autobiography that after spending so long bedridden believing he would be crippled, he felt compelled to "prove to myself that I could do anything most others could do",[1] and viewed the Army as a way of achieving this.

After completing basic training at Trentham Military Camp Rennie was first posted to the Royal New Zealand Artillery, but did not enjoy it and was "first in the queue"[2] to transfer to the Permanent Staff Instructors.

Rennie observed that he, "must have been the youngest to have done so and, I am sure, with the most curious assortment of ranks—I was a substantive lance-corporal, temporary sergeant, acting warrant officer first class.

"[3] 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.

He was posted to 23rd Battalion and commanded a Vickers MG platoon, seeing action in the river-crossing the 2nd Division conducted as it pushed northwards through Italy in the final months of the war in Europe.

For courage, leadership and self discipline in command of the Special Air Service Squadron from New Zealand, during two years of successful jungle warfare against communist bandits.The Queen has been graciously pleased to approve as on the 30th August, 1957, that the following be Mentioned in recognition of gallant and distinguished conduct in operations in Malaya during the period ending on that date.