How the Bromance Between Elon Musk and Sam Altman Turned Toxic

Sam Altman got his first glimpse of Elon Musk’s boundless ambition more than a decade ago when he showed up at SpaceX’s headquarters in Southern California to meet the man who would change his life.

Altman, now chief executive of artificial-intelligence pioneer OpenAI, was then in his 20s and had recently sold his first startup for a disappointing sum. Musk was 14 years his senior and planning to send rockets to Mars. “I left thinking, ‘huh, so that’s the benchmark for what conviction looks like,’” Altman later wrote.

Recently, Altman got a...