The four Beatles—John Lennon, Paul McCartney, George Harrison and Ringo Starr—changed music forever in a relatively brief timespan, bursting onto the scene in 1963 with "Please Please Me" and recording their last albums, "Let It Be" and "Abbey Road," in 1969. The foursome came together as teenagers and became superstars by their early 20s, sharing a unique set of experiences and, naturally, forming a unique set of rivalries and grievances. But why did the most influential band of the 20th century break up within seven years of releasing their first album? The answer is as complex as the relationships between the men themselves.

Many trace the breakup of the Beatles to the death of their manager, Brian Epstein, on August 27, 1967. A record store owner with no experience managing bands, Epstein had nonetheless played a crucial role in their rise to worldwide fame. Epstein also co-founded Northern Songs Ltd. to publish the Beatles’ music, giving Lennon and McCartney each a 15 percent stake.

Exhausted from their extensive tours, during which they couldn’t hear themselves play over the roar of their fans, the Beatles decided to stop performing live in 1966. Epstein opposed the decision, which the foursome felt was necessary in order to focus on the quality of their music. Over the next few years, the Beatles redefined pop music with the seminal "Sgt. Pepper’s Lonely Hearts Club Band" and became global countercultural icons. But they also made less money, and drifted away from Epstein.

When Epstein died of a drug overdose, the Beatles lost a man who expertly managed both their finances and their egos. While Lennon, Harrison and Starr wanted Allen Klein, manager of the Rolling Stones, to take over, McCartney favored Lee and John Eastman, the father and brother of his soon-to-be wife Linda.

“I think that was Paul’s giant miscalculation,” says Tim Riley, a journalism professor at Emerson College and author of several books about the Beatles. “Consider the reverse—he would never have let John Lennon bring his in-laws [into the business].”

Without the reliable income that touring had provided, the Beatles became increasingly desperate for revenue. The "Magical Mystery Tour" film blew minds, but didn’t break any box office records. They ventured into retail with the psychedelic Apple Boutique, but it closed after eight months at a loss of around £200,000.

Over the course of 1968, ’69 and ’70, the Beatles spent hundreds of hours in often contentious business meetings at the headquarters of their label, Apple Records. Financial concerns even found their way into their music: the "Abbey Road" song, “You Never Give Me Your Money,” for example, describes a love affair as a “negotiation” during which both partners “break down.”

The group’s eventual dissolution came during one such appointment. Lennon officially told McCartney and Starr that he was leaving the group during a meeting with Klein on September 20, 1969. Klein persuaded Lennon to keep his departure a secret, so as not to scuttle a lucrative deal that would give Apple ownership of their entire back catalogue. The Beatles signed the deal, which greatly improved their financial situation, on the day that Lennon left the band for good.

Throughout the "Let It Be" sessions in late 1968, Yoko Ono was constantly at Lennon’s side. Lennon had met the conceptual artist in 1966, and by 1969 they had become inseparable, with Ono greatly influencing Lennon’s songwriting and even appearing on several Beatles tracks. Ono’s presence and the surrealist, experimental direction in which she pushed him have led many observers, from the 1960s to the present day, to conclude that Ono was responsible for breaking up the Beatles.

In recent years, however, an increasing number of critics have argued that may not be an adequate explanation.

“I think this idea that Yoko breaks up the band is one of the most racist, insidious, stupidest small-minded things you could possibly say,” says Riley. He asserts that Lennon himself “hid behind” this idea in order to distance himself from the group.

Per Riley, bringing his girlfriend into the group sent the message that Lennon was moving beyond the partnership he had formed with them. Ono served as a lightning rod for their frustrations with Lennon, obscuring other tensions like the financial situation, creative differences and Lennon’s worsening heroin addiction.

In April of 1969, shortly after the end of the sometimes fractious "Let It Be" sessions, Lennon arrived at McCartney’s house eager to work on a new song. One might have expected McCartney to turn up his nose at the composition, titled “The Ballad of John and Yoko,” in which Lennon bemoans the very media attention he and Ono courted while flat-out comparing himself (not for the first time) to Jesus Christ. Instead, he helped Lennon finish the song. The two then rushed to their Abbey Road studio, where they recorded the entire thing in a single evening.

“I was happy to help,” McCartney later wrote. It’s quite a good song; it has always surprised me how with just the two of us on it, it ended up sounding like the Beatles.”

Both Lennon and McCartney would eventually use Ono as an easy explanation for the breakdown of their partnership. (Lennon told Rolling Stone in 1971, "I had to either be married to them [the band] or Yoko, and I chose Yoko.") But, having met when they were 16 and 15, respectively, the two shared a famously complex relationship, and their rivalry predated Ono’s arrival.

In the beginning, every Beatles song was a true collaboration between Lennon and McCartney. Even as they grew up and grew apart, Lennon and McCartney were known to lovingly mimic each other’s style, and they would seldom complete a song without taking some input from the other. Nevertheless, resentments grew between them, with Lennon belittling McCartney’s more old-fashioned songs (he referred to “Ob-La-Di Ob-La-Da” as “granny music”) while McCartney tried to assert himself as the group’s leader. Lennon often complained that crowd-pleasing McCartney compositions like “Hello, Goodbye” were chosen as the A-side of singles, while Lennon’s more introspective works (in this case, “I Am the Walrus”) were relegated to the B-side.

The animosity between them became public in the wake of the Beatles’ breakup. McCartney took a shot at Lennon and Ono on his song “Too Many People,” leveling the charge that Lennon “took [his] lucky break and broke it in two.” Lennon responded, rather more viciously, with “How Do You Sleep?” “The only thing you ever done was ‘Yesterday,’ and since you’re gone you’re just ‘Another Day,’” Lennon sang, comparing a recent McCartney single unfavorably to his Beatles classic.

It was during the promotion of "McCartney," his solo debut and the first album he had ever written without Lennon, that McCartney told the world of the Beatles' breakup. Though Lennon had left the group the previous September, his departure remained a secret until April 10, 1970, when McCartney stated in a promotional “self-interview” that his partnership with Lennon was over. Lennon later denied that he had left the band, placing the blame on McCartney.

Despite the tensions that lurked just beneath the surface, "Let It Be" and "Abbey Road" contain numerous examples of Lennon, McCartney and the other Beatles working in perfect harmony. “Two of Us” may have been inspired by car rides Paul took with Linda, but it’s hard not to imagine that he and John were speaking to each other as they sang of “memories longer than the road that stretches out ahead.” “I’ve Got a Feeling,” a McCartney track, felt incomplete until the incorporation of Lennon’s unfinished “Everybody Had a Hard Year.”

Lennon reportedly requested that his songs be placed on one side of "Abbey Road," with McCartney’s on the other, but it was the blending of their music that made the album iconic. The medley on the second half of "Abbey Road" contains a mix of Lennon and McCartney songs, each flowing into the next. The climactic track, “The End,” demonstrates a singular balance, as the only Ringo Starr drum solo ever included on a Beatles album leads into a three-part guitar solo split between Lennon, McCartney and Harrison.

“The music is full of great affection, high spirits, and also really complicated, knotty emotional issues,” Riley says of the Beatles’ final albums. “Bands are not simple, so I think we should mistrust the simple explanations.”