Despite having rarely left the president’s side over the preceding few months, Musk was blindsided by the announcement, according to people familiar with the matter.

Elon Musk has recently been in the news for his vain attempt to buy the nonprofit that controls OpenAI, but that’s not all.

After helping President Trump get elected, Elon Musk was poised to dominate the country’s AI policies. Then Mr. Altman ...