As we can see in the main article on Hanley, there is disagreement about his date of birth, his place of birth, and his age when he ran away to see. There is also some disagreement about his war service.
He “ran away” to sea in 1915, during the Great War. The ODNB says that “At the age of twelve he left school and joined the merchant navy, serving in a submarine during the First World War.” He actually had a job in Liverpool for four years between school and the navy. But there is a clear nonsense in the ODNB entry: if he joined the merchant navy he was not in a submarine (since all submarines were Royal Navy). He was certainly at sea, but perhaps did not serve in a war ship at all.
If the sources are not agreed on his age (the ODNB and DIB have to insist that he lied about his age to enlist, since they believe he was born in 1901), at least they agree that in 1917, when in Canada in New Brunswick, he jumped ship in order to join the Canadian army. He saw service in France, with the Canadian Black Watch; in summer 1918, while fighting near Bapaume, he was gassed and invalided out of the army.
The more recent biographical research recorded in Wikipedia says that after hospitalisation, he returned to Canada, to Toronto, for two months, and was there demobbed. He may have gone to sea again in 1919 before returning to Liverpool.
My sources, so far, are as for the main entry under Hanley.
Science Fiction and Fantasy Writers in the Great War 