President Donald Trump said Saturday the U.S. wouldn’t endorse the final communiqué of the Group of Seven industrial nations summit and threatened to impose auto tariffs in a statement on Twitter in which he insulted the Canadian prime minister.

Mr. Trump’s unprecedented refusal threw the final hours of the summit, which had ended on a positive note with the joint communiqué, into disarray, raising the specter of an escalating trade battle between allies after what appeared to be a cooling of tensions.

Hours...