The War Requiem was commissioned for the reconsecration of Coventry Cathedral on 30 May 1962 after the original fourteenth century structure was destroyed in a World War II bombing raid on the night of 14 November 1940. The reconsecration was an occasion for an arts festival, for which Michael Tippett also wrote his opera King Priam, which premiered in Coventry the night before the War Requiem.

As a pacifist, Britten was inspired by the commission, which gave him complete freedom to choose the type of music to compose. He conceived of setting the traditional Latin Mass for the Dead interwoven with nine poems about war by the English poet Wilfred Owen. Owen, who was born in 1893, was serving as the commander of a rifle company when he was killed in action on 4 November 1918 during the crossing of the Sambre-Oise Canal in France, just one week before the Armistice. Although he was virtually unknown at the time of his death, he has subsequently come to be revered as one of the great war poets.

Philip Reed has discussed the progression of Britten's composition of the War Requiem in the Cambridge Music Handbook publication on the work. Britten himself acknowledged the stylistic influence of the Requiems of other composers such as Giuseppe Verdi's Requiem on his own composition.

The work was dedicated to four individuals, Roger Burney, Piers Dunkerley, David Gill, and Michael Halliday. Burney and Halliday were friends of Pears and Britten, respectively, who died in the war. According to the Britten-Pears Foundation website, Dunkerley, "one of Britten’s closest friends, took part in the 1944 Normandy landings. Unlike the other dedicatees, he survived the war but committed suicide in June 1959, two months before his wedding." None of the other dedicatees have known graves, but are commemorated on memorials to the missing.

The musical forces are divided into three groups that alternate and interact with each other throughout the piece, finally fully combining at the end of the last movement. The soprano soloist and choir are accompanied by the full orchestra, the baritone and tenor soloists are accompanied by the chamber orchestra, and the boys' choir is accompanied by a small portative organ (this last group ideally being situated at some distance from the full orchestra). This group produces a very strange, distant sound. The soprano and choir, as well as the boys` choir sing the traditional Latin Requiem text while the tenor and baritone sing poems by Wilfred Owen, interspersed throughout.

The full orchestra consists of three flutes (third doubling piccolo), two oboes, cor anglais, three clarinets (third doubling E-flat clarinet and bass clarinet), two bassoons and contrabassoon, six horns, four trumpets in C, three trombones, tuba, timpani, percussion (four players: two crotales, glockenspiel, tamtam, tubular bells, vibraphone, cymbals, triangle, castanets, temple block, whip, bass drum, two snare drums, tambourine, and tenor drum), piano, portable organ or harmonium (a grand organ is called for only in the Libera Me, the last movement), and strings.

The chamber orchestra consists of flute doubling piccolo, oboe doubling cor anglais, clarinet, bassoon, horn, percussion (one player: timpani, tam-tam, cymbals, bass drum, and snare drum), harp, and string quintet (standard string quartet and double-bass).

The work consists of six movements:

One striking juxtaposition is found in the Offertorium, a fugue in the repeating three-part time scheme 6/8, 9/8, 6/8 where the choir sings of God's promise to Abraham ("Quam olim Abrahae promisisti, et semini eius" — "which you once promised Abraham and his seed"). This frames Owen's retelling of the offering of Isaac, in which the angel tells Abraham to:

But the old man would not so, but slew his son,and half the seed of Europe, one by one.