‘At U.N. headquarters, staffers walked around in a daze of disbelief,” reported a writer for the New Yorker magazine in spring 2005. The cause of this anguish was news that then-President George W. Bush had nominated John Bolton to be America’s next U.N. ambassador. The hawkish, pugnacious Mr. Bolton, the New Yorker writer argued, epitomized the Bush administration’s dangerous disregard for world opinion.

That New Yorker writer was Samantha Power, and in 2013 President Barack Obama nominated the Harvard academic to the same...