Roberts, promoted to lieutenant on 3 September 1914, along with his battalion, then commanded by Lieutenant Colonel George Grogan, returned to the United Kingdom shortly after the declaration of war and arrived there in October 1914, where it became part of the 24th Brigade of the 8th Division, which was then in the process of formation. The battalion was sent to reinforce the British Expeditionary Force (BEF) on the Western Front and landed in France in November 1914. The battalion was involved in the winter operations 1914–1915 and participated in the Christmas truce. Three months after his arrival, Roberts was awarded the Distinguished Service Order while leading a trench raid. He was made his battalion's adjutant and fought in the Battle of Neuve Chapelle, and was promoted to captain on 6 August 1915 He remained as adjutant until October 1916 when he was made a brigade major of the 8th Division's 23rd Brigade. In this capacity, during the Battle of Passchendaele (also known as the Third Battle of Ypres) in August 1917, Roberts was awarded the Military Cross (MC). On 24 October 1917 he returned to the 1st Battalion, Worcesters and, aged just twenty-six, was promoted to the acting rank of lieutenant-colonel, and given temporary command of the battalion, becoming one of the British Army's youngest battalion commanders.