[2] In 1941, after undertaking officer training, he was commissioned with the rank of lieutenant and assigned to the 2/32nd Battalion,[2] of the 9th Division,[4] serving with them during the North African campaign and then later in New Guinea after they were brought back to fight against the Japanese.

[5] Following the end of the war, Denness transferred to the 66th Battalion and deployed to Japan as part of the British Commonwealth Occupation Force, where he commanded the battalion's 'B' Company, carrying out a number of operations including a search raid on the village of Kinoe where Denness' company discovered a contraband smuggling vessel.

On reaching a position approximately one mile from the Airborne Division, his company came under heavy rifle and machine-gun fire from a wooded ridge to his front and right flank.

Despite heavy fire from three sides, Captain Denness resolutely pressed home his original attack, accounting for seventy-five enemy dead.

His calm, resolute action was an inspiration to the entire company and ensured an early junction with the forward elements of 11 US Airborne Division who were in desperate need of support.