Conventional understanding/speculation has been that Yoko Ono was the cause of The Beatles' break-up. Her inclusion in their creative process and her unwanted participation was supposed to have led to a rift between John Lennon and the rest of the group.
A newly released Letters of Note seems to show that the rift, at the end of the group's collaboration, was between Paul McCartney and the other band members.
Letters of Note describes them as "two letters that perfectly illustrate the rift that ultimately tore The Beatles apart following the death of manager Brian Epstein and Apple Corps' subsequent failings. First, the draft of an undated letter in Lennon's hand that essentially bars Paul McCartney and his new manager, Lee Eastman, from accessing The Beatles' recordings without authorisation; followed by a 1969 letter to Eastman specifically, signed by Lennon, Harrison and Starr, in which he is reminded in no uncertain terms that, despite his managing of McCartney, he has no control over the affairs of the band."
You can see the actual letters and transcipts here.