Isaac returned to Britain in 1915.
He was was unable to make a living, so, despite his criticisms of the war, he enlisted in the army and tried to join the medical services.
However, he was judged too short for regular regiments and so was sent
to a 'Bantam' battalion (for men under five feet three inches
tall) called the 12th Suffolk Regiment. Reportedly, Isaac
was unsuited to Army life, being clumsy and absent minded,
and he also encountered anti-semitism from officers and soldiers
alike. In a letter to his friend Schiff dated
October 1915, Isaac wrote:
"I don't mind the hard sleeping the stiff
marches etc. but this is unbearable. Besides my being a Jew makes
it bad amongst these wretches."