On 21 October 1915, he enlisted in the Artists' Rifles Officers' Training Corps. For the next seven months, he trained at Hare Hall Camp in Essex. On 4 June 1916 he was commissioned as a second lieutenant (on probation) in the Manchester Regiment. Initially, he held his troops in contempt for their loutish behaviour, and in a letter to his mother described his company as "expressionless lumps". However, his imaginative existence was to be changed dramatically by a number of traumatic experiences. He fell into a shell hole and suffered concussion; he was blown high into the air by a trench mortar, and spent several days lying out on an embankment in Savy Wood amongst (or so he thought) the remains of a fellow officer. Soon afterwards, Owen was diagnosed as suffering from neurasthenia or shell shock and sent to Craiglockhart War Hospital in Edinburgh for treatment. It was while recuperating at Craiglockhart that he met fellow poet Siegfried Sassoon, an encounter that was to transform Owen's life.
Whilst at Craiglockhart, he made friends in Edinburgh's artistic and literary circles, and did some teaching at the Tynecastle High School, in a poor area of the city. In November he was discharged from Craiglockhart, judged fit for light regimental duties. He spent a contented and fruitful winter in Scarborough, North Yorkshire, and in March 1918 was posted to the Northern Command Depot at Ripon. While in Ripon he composed or revised a number of poems, including "Futility" and "Strange Meeting". His 25th birthday was spent quietly at Ripon Cathedral, which is dedicated to his namesake, St. Wilfrid of Hexham.
In July 1918, Owen returned to active service in France, although he might have stayed on home-duty indefinitely. His decision was probably the result of Sassoon's being sent back to England, after being shot in the head in an apparent "friendly fire" incident, and put on sick-leave for the remaining duration of the war. Owen saw it as his duty to add his voice to that of Sassoon, that the horrific realities of the war might continue to be told. Sassoon was violently opposed to the idea of Owen returning to the trenches, threatening to "stab [him] in the leg" if he tried it. Aware of his attitude, Owen did not inform him of his action until he was once again in France.
At the very end of August 1918, Owen returned to the front line - perhaps imitating the example of his admired friend Sassoon. On 1 October 1918 Owen led units of the Second Manchesters to storm a number of enemy strong points near the village of Joncourt. For his courage and leadership in the Joncourt action, he was awarded the Military Cross, an award he had always sought in order to justify himself as a war poet, but the award was not gazetted until 15 February 1919. The citation followed on 30 July 1919:
2nd Lt, Wilfred Edward Salter Owen, 5th Bn. Manch. R., T.F., attd. 2nd Bn.
For conspicuous gallantry and devotion to duty in the attack on the Fonsomme Line on October 1st/2nd, 1918. On the company commander becoming a casualty, he assumed command and showed fine leadership and resisted a heavy counter-attack. He personally manipulated a captured enemy machine gun from an isolated position and inflicted considerable losses on the enemy. Throughout he behaved most gallantly.