During the Battle of the Somme in September 1916, Dalton was involved in bloody fighting during the Battle of Ginchy, in which over 4,000 Irishmen were killed or wounded. Among the casualties was Tom Kettle, a former nationalist Member of Parliament and personal friend of Dalton's father and of Emmet. Dalton was awarded the Military Cross for his conduct in the battle. Afterwards he was transferred to the 6th Battalion, Leinster Regiment, and sent to Salonika then Palestine, where he commanded a company and then supervised a sniper school in El Arish. In 1918 Dalton was re-deployed again to France, and in July promoted to captain, serving as an instructor.