The Senate on Friday confirmed Michael Waltz, the former national security adviser, to serve as the top United States diplomat to the United Nations. All but two Democrats opposed Mr. Waltz’s nomination, arguing that he had shown himself to be unfit to hold a top foreign policy post when it was revealed earlier this year that he had discussed war plans with top officials in a Signal chat to which he had inadvertently invited a journalist — and then was unwilling to admit the mistake. He was confirmed on a 47-to-43 vote, with Democratic Senators Jeanne Shaheen of New Hampshire,...