War and Peace (Op. 91) (Russian: Война и мир, Voyna i mir) is a 1946 230-minute opera in 13 scenes, plus an overture and an epigraph, by Sergei Prokofiev. Based on the 1869 novel War and Peace by Leo Tolstoy, its Russian libretto was prepared by the composer and Mira Mendelson. The first seven scenes are devoted to peace, the latter six, after the epigraph, to war.

Although Tolstoy's work is classified as a novel, the 1812 invasion of Russia by the French was a historical event, and some real-life people appear as characters in both the novel and the opera, e.g. Prince Mikhail Kutuzov and Napoleon Bonaparte.

Mendelson and Prokofiev's original scheme for the libretto of the opera envisaged 11 scenes, and Prokofiev began composing the music in the summer of 1941, spurred on by the German invasion of the Soviet Union which began on June 22, 1941. The description "lyric-dramatic scenes" in the libretto accurately suggests both a homage to Tchaikovsky's Eugene Onegin and an emphasis on individuals and their emotions rather than on the bigger picture of a country at war.

A piano score was completed by the summer of 1942 (two scenes having been changed from the original version), and it was submitted to the Soviet Union's Committee on the Arts. The Committee demanded that the Part 2 (War) scenes needed a more patriotic and heroic emphasis. Prokofiev, who had wanted to see his masterpiece staged as quickly as possible, added marches, choruses, and other materials to Part 2 to satisfy the committee. In addition, he composed the choral Epigraph, which emphasises the Russian people's defiance in the face of the enemy.

Plans were drawn up for a 1943 première at the Bolshoi Theatre, Moscow, to be directed by Sergei Eisenstein and conducted by Samuil Samosud. Nothing came of this project, although a private performance of eight scenes with piano accompaniment took place at the Moscow Actors’ Centre on October 16, 1944, and a public concert performance of nine scenes, conducted by Samosud, was given in the Great Hall of the Moscow Conservatory on June 7, 1945.

The first staged performance was of a newly extended seven-scene version of Part 1 (what is now Scene 2 having been added at Samosud's suggestion), together with Scene 8, the first scene of Part 2. This took place on June 12, 1946, at the Maly Theatre (before the Revolution – Mikhailovsky Theatre) in Leningrad, again conducted by Samosud. Part 2, also with an additional scene (Scene 10), was to be performed there in July 1947, but after the dress rehearsal no public performances were given, “for reasons beyond the control of the theatre and the composer”.

Following the Zhdanov decree of February 1948, Prokofiev started work on a shortened single-evening version of the opera, at the same time making various revisions to his original scheme, although the thirteen-scene framework remained. This version was first performed on 26 May 1953, at the Teatro Comunale, Florence, conducted by Artur Rodziński, two months after the composer's death. Scenes 2 and 9 were, however, omitted. The Russian première of this version was given at the Maly Theatre, Leningrad, on April 1, 1955, conducted by Eduard Grikurov, in this case with the omission of Scenes 7 and 11. All thirteen scenes (but with cuts) were eventually first performed together on November 8, 1957, at the Stanislavski-Nemirovich-Danchenko Theatre in Moscow, under the baton of Samosud's assistant Alexander Shaverdov. On December 15, 1959, the 13 scenes and Epigraph were finally staged uncut (conducted by Alexander Melik-Pashayev) at the Bolshoi Theatre, Moscow, although this was preceded in the United States by an NBC telecast conducted by Peter Herman Adler on January 13, 1957.

The first British performance was a Leeds Festival concert performance of nine scenes at Leeds Town Hall on April 19, 1967 conducted by Edward Downes. The first British staged performance was by Sadlers’ Wells Opera on October 11, 1972 in English in a production by Colin Graham conducted by David Lloyd-Jones, and the first American staging by the Opera Company of Boston on May 8, 1974. In other countries, the thirteen-scene version of the opera was first performed in Germany (Bonn) and Bulgaria (Sofia) in 1957, Serbia (Belgrade) in 1958, Croatia (Zagreb) in 1961, the Czech Republic (Liberec) in 1962, France (Théâtre des Champs-Élysées, Paris, in concert) and Canada (Montreal) in 1967, Austria (Vienna State Opera, conducted by Mstislav Rostropovich) in 1971, Australia (the opening performance at the Sydney Opera House) and Argentina (Teatro Colón, Buenos Aires) in 1973, Spain (Liceu, Barcelona) in 1977 and the Netherlands (Amsterdam, conducted by Edo de Waart) in 1991. The Canadian Opera Company performed the opera as part of its 2008–09 season.