Britain's King Charles III speaks during a joint address to Congress in the House Chamber at the U.S. Capitol in Washington on Tuesday. | Bloomberg

Britain's King Charles III called on the U.S. to maintain its leadership role in the world and to support NATO and Ukraine, in a remarkably pointed speech for a monarch who urged Americans to resist the pull of isolationism.

Charles used his remarks to a joint session of Congress — the first such address by a British monarch in more than three decades — to argue for preserving the alliance between the U.K. and U.S.…