At the start of WW1, Isaac was employed as a cabinet maker with D. Buckle Ltd, an established London-based furniture company who were contracted to produce ammunition boxes for the British Army. At that time, his job was considered a reserved occupation.

We don't know when Isaac enrolled in the Army. He still held Russian nationality as he didn't apply for British citizenship until some years later. He served in France in the 7th Royal Sussex Regiment and the Prince of Wales Own, the 5th West Yorkshire Regiment.

Sadly his mother Ann died in December 1918 and when Isaac returned to London in 1919, he found a new marriage had already been arranged for his very religious father, the headmaster of a cheder and a Board member of Old Castle Street Synagogue. Isaac was uncomfortable with this and he transferred to the RASC (Royal Army Service Corps) in July 1919, becoming a corporal in the Army of Occupation. He was demobilised in September 1920 when he received £15 19s 10d.

He was based for at least part of this service in Cologne (now Kohn) in Germany which allowed him to indulge in his passion for opera by attending as many performances as he could, including some with the famous conductor Otto Klemperer.

Many years later, when he heard that Klemperer was to perform in London, there was some correspondence between him and Klemperer's daughter and some programmes he collected during his time with the army were sent to the conductor.