Rosenberg was assigned to the 12th Battalion of the Suffolk Regiment, a bantam battalion for men under the usual minimum height of 5'3". After turning down an offer to become a lance corporal, Private Rosenberg was later transferred to another bantam battalion, the 11th (Service) Battalion of The King's Own Royal Lancaster Regiment. In June 1916, he was sent with his Battalion to serve on the Western Front in France. He continued to write poetry while serving in the trenches, including Break of Day in the Trenches, Returning we Hear the Larks, and Dead Man's Dump.
Having just finished night patrol, he was killed at dawn on 1 April 1918; there is a dispute as to whether his death occurred at the hands of a sniper or in close combat. In either case, he died in a town called Fampoux, north-east of Arras. He was first buried in a mass grave, but in 1926, his remains were identified and reinterred at Bailleul Road East Cemetery, Plot V, Saint-Laurent-Blangy, Pas de Calais, France.