stylistic:style_transfer, structural:scholar_edition
Studio years, break-up and solo work: 1966–1970

After the band's final concert on 29 August 1966, Lennon filmed the anti-war black comedy How I Won the War – his only appearance in a non-Beatles feature film – before rejoining his bandmates for an extended period of recording, beginning in November. Lennon had increased his use of LSD and, according to author Ian MacDonald, his continuous use of the drug in 1967 brought him "close to erasing his identity ". The year 1967 saw the release of " Strawberry Fields Forever ", hailed by Time magazine for its "astonishing inventiveness", and the group's landmark album Sgt. Pepper's Lonely Hearts Club Band, which revealed lyrics by Lennon that contrasted strongly with the simple love songs of the group's early years.

In late June, the Beatles performed Lennon's " All You Need Is Love " as Britain's contribution to the Our World satellite broadcast, before an international audience estimated at up to 400 million. Intentionally simplistic in its message, the song formalised his pacifist stance and provided an anthem for the Summer of Love. After the Beatles were introduced to the Maharishi Mahesh Yogi, the group attended an August weekend of personal instruction at his Transcendental Meditation seminar in Bangor, Wales. During the seminar, they were informed of Epstein's death. "I knew we were in trouble then", Lennon said later. "I didn't have any misconceptions about our ability to do anything other than play music. I was scared – I thought, 'We've fucking had it now. ' " McCartney organised the group's first post-Epstein project, the self-written, -produced and -directed television film Magical Mystery Tour, which was released in December that year. While the film itself proved to be their first critical flop, its soundtrack release, featuring Lennon's Lewis Carroll –inspired " I Am the Walrus ", was a success.

Led by Harrison and Lennon's interest, the Beatles travelled to the Maharishi's ashram in India in February 1968 for further guidance. While there, they composed most of the songs for their double album The Beatles, but the band members' mixed experience with Transcendental Meditation signalled a sharp divergence in the group's camaraderie. On their return to London, they became increasingly involved in business activities with the formation of Apple Corps, a multimedia corporation composed of Apple Records and several other subsidiary companies. Lennon described the venture as an attempt to achieve "artistic freedom within a business structure". Released amid the Protests of 1968, the band's debut single for the Apple label included Lennon's B-side " Revolution ", in which he called for a "plan" rather than committing to Maoist revolution. The song's pacifist message led to ridicule from political radicals in the New Left press. Adding to the tensions at the Beatles' recording sessions that year, Lennon insisted on having his new girlfriend, the Japanese artist Yoko Ono, beside him, thereby contravening the band's policy regarding wives and girlfriends in the studio. He was especially pleased with his songwriting contributions to the double album and identified it as a superior work to Sgt. Pepper. At the end of 1968, Lennon participated in The Rolling Stones Rock and Roll Circus, a television special that was not broadcast. Lennon performed with the Dirty Mac, a supergroup composed of Lennon, Eric Clapton, Keith Richards and Mitch Mitchell. The group also backed a vocal performance by Ono. A film version was released in 1996.

By late 1968, Lennon's increased drug use and growing preoccupation with Ono, combined with the Beatles' inability to agree on how the company should be run, left Apple in need of professional management. Lennon asked Lord Beeching to take on the role but he declined, advising Lennon to go back to making records. Lennon was approached by Allen Klein, who had managed the Rolling Stones and other bands during the British Invasion. In early 1969, Klein was appointed as Apple's chief executive by Lennon, Harrison and Starr but McCartney never signed the management contract.

Lennon and Ono were married on 20 March 1969 and soon released a series of 14 lithographs called "Bag One" depicting scenes from their honeymoon, eight of which were deemed indecent and most of which were banned and confiscated. Lennon's creative focus continued to move beyond the Beatles, and between 1968 and 1969 he and Ono recorded three albums of experimental music together: Unfinished Music No. 1: Two Virgins (known more for its cover than for its music), Unfinished Music No. 2: Life with the Lions and Wedding Album. In 1969, they formed the Plastic Ono Band, releasing Live Peace in Toronto 1969. Between 1969 and 1970, Lennon released the singles "Give Peace a Chance", which was widely adopted as an anti- Vietnam War anthem, " Cold Turkey ", which documented his withdrawal symptoms after he became addicted to heroin, and " Instant Karma! ".

In protest at Britain's involvement in "the Nigeria-Biafra thing" (namely, the Nigerian Civil War), its support of America in the Vietnam War and (perhaps jokingly) against "Cold Turkey" slipping down the charts, Lennon returned his MBE medal to the Queen. This gesture had no effect on his MBE status, which could be renounced but ultimately only the Sovereign has the power to annul the original award. The medal, together with Lennon's letter, is held at the Central Chancery of the Orders of Knighthood.

Lennon left the Beatles on 20 September 1969, but agreed not to inform the media while the group renegotiated their recording contract. He was outraged that McCartney publicised his own departure on releasing his debut solo album in April 1970. Lennon's reaction was, "Jesus Christ! He gets all the credit for it!" He later wrote, "I started the band. I disbanded it. It's as simple as that." In a December 1970 interview with Jann Wenner of Rolling Stone magazine, he revealed his bitterness towards McCartney, saying, "I was a fool not to do what Paul did, which was use it to sell a record." Lennon also spoke of the hostility he perceived the other members had towards Ono, and of how he, Harrison and Starr "got fed up with being sidemen for Paul... After Brian Epstein died we collapsed. Paul took over and supposedly led us. But what is leading us when we went round in circles?"