In February 2021, two weeks after he moved into the White House, President Biden reminded the nation’s diplomats of his promise to restore American leadership in the world. In a speech at the State Department, he summed up his election in three words: “America is back.” On Tuesday, Mr. Biden will confront the limits of that promise when he addresses world leaders at the United Nations for his fourth and final time as president. The White House says he will make the case that his “...