Campion was commissioned into the part-time 2nd Volunteer Battalion, Royal Sussex Regiment, later 4th Battalion, Royal Sussex Regiment (of which his father was Honorary Colonel) in 1888. On the outbreak of World War I he was the battalion's senior Captain with the rank of honorary Major. He became Lieutenant-Colonel of the 1st Line battalion (1/4th Royal Sussex) and commanded it in the Gallipoli campaign. At its first action, at Suvla Bay on 9 August 1915, the "Official History" records that the orders given to Lt-Col Campion 'were vague in the extreme. The colonel was verbally told "to restore the line". No one knew where it was, but he was told that "if he went in that direction (pointing to a column of smoke from the burning scrub) he ought to find the &lt;a href="4th%20Battalion%2C%20Queen%27s%20Royal%20Regiment%20%28West%20Surrey%29%232/4th%20Battalion"&gt;2/4th Queen's&lt;/a&gt;".' The 1/4th Sussex advanced steadily in extended order, found the Queen's who had been driven off the crest, and consolidated a line, having suffered heavy casualties. Campion was evacuated to England sick on 5 October.