What caused The Beatles to break up?

The primary reason was the death of manager Brian Epstein in 1967, which removed the band’s mediator. The Get Back/Let It Be sessions, Apple Corps financial disaster, and the McCartney vs. Lennon power struggle did the rest. Yoko Ono was a convenient scapegoat, not the cause—she accelerated Lennon’s exit, but the fractures were already there. The breakup was a slow bleed that started when they stopped touring in 1966.