What if the U.S. had taken a more active and constructive role in international affairs after the First World War, rather than reject the Treaty of Versailles and refuse to join the League of Nations? In the view of historian Robert Kagan, another global conflict would have been avoided, and Adolph Hitler might never have been appointed German chancellor as he was in January 1933. This is the subject of Kagan's latest book, "The Ghost at the Feast," and in this episode, he defends his thesis concerning the importance of U.S. leadership, or its absence, after the seismic shifts in global power caused by the war of 1914-18. As Kagan, a senior fellow at the Brookings Institution, put it in a recent essay in Foreign Affairs about U.S. support for Ukraine, "Only American power can keep the natural forces of history at bay." Is that true today? Was it true between 1919 and 1939?