The 4th Black Watch were sent to France in 1915, and in that year took part in the battles of Aubers Ridge, Neuve Chapelle and Loos. During his time fighting, Lee sent sketches and poems back home to Scotland and became known as 'the Black Watch Poet'. These poems were eventually collected in two books of poetry, "Ballads of Battle" and "Work-a-Day Warriors". In 1917 he gained a commission as a second lieutenant in the 10th Battalion of the King's Royal Rifle Corps. Later that year he was reported to be missing in action. In fact Lee had been captured and became a prisoner of war in Germany where he was held at camps at Karlsruhe and Beeskow. During his imprisonment, Lee kept journals in which he included sketches and other material. These journals were adapted into "A Captive in Carlsruhe", a book which chronicled his time as a POW published in 1920.