Paul John Esser German Born but served in British Regiment

[Dragon2009]

My grandfather Paul John Esser b 1888-d 1970 was born in Koln Germany. It seems his family moved to England when he was a toddler.
It appears from the 1939 register and from other records that he served with the Royal Field Artillery for 12 years incorporating WWI.
My question is that as he was German born was it unusual to serve in a British regiment especially at time of war ??
Although he was my grandfather I never met him as I was adopted as a baby
Thanks for any info

[CaroleW]

Welcome to Rootschat

Did he become a British Citizen?  Were his parents Otto & Anna as they are indexed as Eggers on the 1901 entry

His Medal card shows he was an acting Sgt & was awarded the British/Victory & 15 star medals

Theatre of War is shown as (5G) (a) and was entered on 29.8.1915