At the outbreak of the 1914 - 18 War, Collet was at Eastchurch in the Eastchurch Wing of the RNAS under Wing Commander Samson. For the first few days they carried out patrols of the North Sea coast but on 27 August 1914 the Wing moved to Ostend with a dozen assorted aeroplanes, BE 2s, Sopwith Biplanes, a Short Biplane, two Bleriot Monoplanes, a Henri Farman and a Bristol Biplane. They found themselves on the exposed left flank of the Allied armies operating from a landing ground at St. Pol near Dunkirk. The unit then became No.1 Naval Wing of the R.N.A.S. and in addition to its aircraft it also had its own makeshift armoured cars to assist in forming temporary bases to extend the range of the aircraft. Apart from covering the flank of the retreating BEF it also had a role in preventing Zeppelins reaching U.K., and being based in Belgium, and also to stop submarines being assembled.

On 22 September 1914 Collet led a raid by four aircraft against the Zeppelin sheds at Dusseldorf and Cologne but because of thick mist in the Rhine valley only Collet found his target, dropping two 20lb bombs from 400 feet on the shed at Dusseldorf. Although he scored directhits the bombs failed to explode but even so Collet had made history as the first man to attack the enemy in this way. He was awarded the D.S.O.
In March 1915 the wing, now 3 Wing RNAS moved from Dunkirk to the Gallipoli theatre and took over an improvised airfield on the island of Tenedos. Apart from reconnaissance and spotting the Turks were bombed on every possible occasion. On 22 June Capt. Collet with Major R.E.T. Hogg as observer flying a Voisin intercepted a German aircraft near Achi Baba and Hogg, and, using a rifle, hit in the engine and shot it down.
It is of interest that Ark Royal II, built in 1914 as the first aircraft carrier in the world, with a forward flying off deck for land planes but equipped as a seaplane carrier, was involved in all the operations in the Gallipoli theatre. The land planes of No.3 Squadron in which Collet served, were commanded by Wing Commander E.L. Gerrard, the first Flying Marine. From dawn to dusk they were continuously over the coast in the area of the Helles landings. Later the squadron moved to a small airfield on Imbros which ended on the edge of a cliff. On 19 August Collet was taking off when his engine failed. As he turned to regain the airfield he was caught in an up - draft, crashed and his machine caught fire. He died before his rescuers could pull him clear - one of them C.P.O. Keogh was badly burned and was later awarded the Albert Medal.

Lt Collet was known as the Marine with a photographic memory so accurate that he could play chess blindfold. An extract from a report from the Aeroplane Squadron, Imbros 21 August 1915 says;- "By the unfortunate death of this officer the Navy and the Air Service lose a most devoted and valuable pilot who worked most tremendously hard with splendid results."