At the outbreak of World War I Bryan was still at school and was too young to enlist. In 1915, aged 17, he joined the Territorial Force, being awarded a commission as a 2nd Lieutenant in the 5th Battalion, the Manchester Regiment, the same battalion his brother Jack was serving in and the brother's father had been a Lieutenant-colonel in. He was initially posted to a Provisional Battalion as he was too young to serve overseas. He was promoted to Lieutenant in 1917 and, in January 1918, posted to the main complement of the battalion, now old enough to serve on the front line.