The outgoing U.S. ambassador to the United Nations, Linda Thomas-Greenfield, warns that if America's leadership diminishes again during Trump's second term, China and other adversaries will fill the vacuum. She emphasizes the importance of U.S. engagement with the world and the need to rebuild alliances and reestablish America's leadership. Thomas-Greenfield stresses that U.S. leadership is crucial as rivals will change the rules of the road. She also acknowledges that the U.N. needs reforms but highlights its importance in dealing with major global issues. Thomas-Greenfield advises Trump's nominee, Rep. Elise Stefanik, to meet quickly with all members of the U.N. Security Council, including Russia and China.