The European Union is the largest economy in the history of the world and the most important zone of contiguous democracies today. What it lacks is a sense of its own history, which creates a surprisingly important opportunity for those who wish it ill, above all in Moscow. Here we try to explain what has made the European Union possible, and what will be necessary to defend its future. Timothy Snyder is a historian at Yale University, specializing in eastern Europe, totalitarianism, and the Holocaust. His books have received widespread acclaim. His most recent book, "The Road to Unfreedom: Russia, Europe, America," tells the whole Russia story, from beginning to end, in a way that makes sense and reveals the big picture. He is also the author of "On Tyranny: Twenty Lessons from the Twentieth Century," which explores the everyday ways a citizen can resist the authoritarianism of today. His other works include "Black Earth: The Holocaust as History and Warning" and "Bloodlands: Europe Between Hitler and Stalin." Follow on Twitter: @TimothyDSnyder