W/Cmdr. Foster was called up at the outbreak of war and joined No 605 Squadron RAFVR on 6th July 1940.
In combat on 27th September he damaged a Bf 110 but suffered damage to his own aircraft and had to make a forced landing. The next day he damaged a Ju 88.
On 7th October he destroyed a Bf 109, and shared a Ju 88 on the following day.
on 26th October and 8th November he managed to score hits and damage Bf 109s.
On 24th March, 1941, he damaged a He 111.

He left the squadron in September 1941 to become an instructor at 55 OTU.

Foster joined 54 Squadron on 14th April 1942, which was posted to Australia and arrived there 7th September 1942.
He scored the squadron’s first victory in Australia with a Mitsubishi ‘Dinah’ on 6th February 1943, followed by four Mitsubishi ‘Bettys’, two more probables and two damaged between 15th March and 6th July.