From the beginning of the First World War Plowman felt morally opposed to the fighting - "insane and unmitigated filth" - but on Christmas Eve 1914 he reluctantly volunteered for enlistment in the Territorial Army 4th Field Ambulance. He later accepted a commission in an infantry regiment, and serving at Albert, close to the Somme on the Western Front, he suffered concussion from an exploding shell. Deemed to be affected by shell shock, he was sent home to convalesce at Bowhill Auxiliary, a branch of Craiglockhart, where he was treated by W. H. R. Rivers, although he did not meet either of Rivers' two most celebrated patients, Wilfred Owen and Siegfried Sassoon. While recovering, he produced a poetry collection, A Lap Full of Seed, and an anonymous pamphlet, The Right to Live, inveighing against the kind of society that made war inevitable. Having been granted a further month's home service in January 1918, he wrote to his battalion adjutant asking to be relieved of his commission on the grounds of religious conscientious objection to all war. He was arrested and tried by court martial for refusing to return to his unit; his trial was covered in the Labour Leader in April 1918. Having been dismissed from the Army, fortunately without punishment, he was served with notice of call-up as a conscript, but successfully applied to a Tribunal for exemption as a conscientious objector.
In July 1918 Plowman gave a positive review in the Labour Leader to Siegfried Sassoon's anti-war poetry collection Counter-Attack. It was in response to a request in a letter from Plowman that Sassoon campaigned for Philip Snowden in Blackburn, in the December 1918 General Election.
His memoir A Subaltern on the Somme of the war was published in 1928, under the pseudonym "Mark VII".
In 1929 George Orwell had sent The New Adelphi an article. Plowman sent Orwell books to review, founding an important friendship; and Rees was Orwell's literary executor. Plowman later got to know Orwell better through Mabel Fierz. Orwell later described Plowman as "pugnacious". Orwell was still in agreement with Plowman's pacifism in early 1938. Later that year Plowman introduced Orwell to Leo Myers, and set up a secret gift of £300 from Myers so that Orwell and his wife could travel to Morocco, to restore Orwell's health.
Plowman co-founded in 1934 and ran the Adelphi Centre. It was an early commune, based on a farm in Langham, Essex bought by Middleton Murry. Short-lived in its original conception, it ran a Summer School in August 1936 that was stellar: Orwell spoke on "An Outsider Sees the Distressed Areas" on 4 August, with Rayner Heppenstall in the chair. Other speakers were Steve Shaw, Herbert Read, Grace Rogers, J. Hampden Jackson, N. A. Holdaway (a Marxist theorist and schoolmaster, and a Director of the Centre), Geoffrey Sainsbury, Reinhold Niebuhr, Karl Polanyi, John Strachey, Plowman and Common..
Through it he also met the pacifist dramatist Richard Heron Ward, who from 1936 became a close friend. Ward formed the 'Adelphi Players' in 1941, who used the Adelphi Centre for rehearsals.
Plowman was attracted into organising for pacifism in the later 1930s by Hugh Richard Lawrie Sheppard. He was the first General Secretary of the Peace Pledge Union 1937-1938. Murry, to whom Plowman was now close, became a pacifist after a diversion into communism.