F/O Secretan joined the RAF on a short service commission in January 1939.
In 1940, Dennis was posted to 54 Squadron at Catterick on 3rd September, and was then moved to 72 Squadron at Biggin Hill on 27th.

On 27th October, Dennis damaged a Bf 110.

He was mentioned in Despatches of 1st January 1941.

In September 1942, Dennis was given command of 242 Squadron at Digby, and took the squadron to North Africa in November.

On 1st January 1943, Dennis damaged a FW 190.
On 2nd January, he destroyed a Ju 87, but as his ammunition exhausted, he was attacked by two Bf 109s and decended rapidy to make a forced landing seven miles north of Morris. The pilots of the pursuing aircraft waited until Dennis had got out and away from his aircraft before machine-gunning it.

Dennis was awarded the DFC on 26th February 1943, and left 242 Squadron a month later.

After returning to the UK he served with Transport Command.

Dennis was released from the RAF in 1946 as Wing Commander.