There's been talk of Ukraine possibly joining NATO since the early years of post-Cold War Europe, but it never happened. And the allies aren't quite ready to go ahead with membership now, as evidenced by their vaguely-worded commitment issued at the Vilnius summit "to extend an invitation to Ukraine to join the Alliance when Allies agree and conditions are met." From the moment the post-Soviet world started coming into view, when and where NATO should expand has aggravated relations between the U.S. and Moscow. When it came to Ukraine, the country got the worst of both worlds: it was left on the wrong side of Europe's dividing line and Russian leaders were angered by the mere idea of Ukraine entering NATO. In this episode, historian Jeffrey Engel discusses the origins of today's debate about Ukraine's future, whose circumstances could compel the U.S. and its European allies into direct conflict with Russia.