Dennis then went to train at nautical school planning to join the British Merchant Navy. He trained aboard the sail training ship HMS Worcester, which was moored off Greenhythe in Kent. The Thames Nautical Training College, as it is now called, was, for over a hundred years, situated aboard ships named HMS Worcester.
Painting of the First boat race on the Mersey between cadets of HMS Conway, which was moored on the River Mersey, and HMS Worcester, 11th June 1891 by Charles William Wyllie (1853 – 1923). The race became an annual event.
During the First World War, Dennis was commissioned as a 2nd Lieutenant into the Royal Field Artillery, receiving his basic training at Biscot Camp in Luton. He was then assigned to the City of London Brigade and the 36th (Ulster) Division and served in Flanders on the Ypres Salient and in France at Cambrai and St. Quentin. Gassed in a chlorine attack during Passchendaele, Dennis was invalided out of the Army.