Soon after the British Empire declared war on Germany on 5August 1914, Dewar signed up to the New Zealand Expeditionary Force, and took part in the expedition to occupy German Samoa, departing the day after he played for Taranaki defending a challenge from Wairarapa. On his return, he volunteered again for active service and enlisted in the Wellington Mounted Rifles, joining the machine gun section. After promotion to sergeant, he departed in October for Egypt. There, he captained his regiment's rugby team in matches against other New Zealand and Australian regiments. He landed at Anzac Cove on 8August 1915 and participated in the action to capture Chunuk Bair in the Gallipoli Campaign. The following day he was killed in action.