At the outbreak of the First World War in August 1914, Rosher was commanding the 2nd Battalion of the Dorsetshire Regiment, which was part of the 16th Indian Brigade stationed in Poona. The battalion was ordered to the Persian Gulf, where they landed at Fao in November 1914 as part of the Indian Expeditionary Force. During the Mesopotamian campaign, Rosher was mentioned in despatches twice, once at the fall of Basra, and again at the Battle of Shaiba. On 14 April 1915, the third day of the latter battle, Rosher was killed in action leading a bayonet charge against Ottoman forces. He is buried at the Basra War Cemetery.