"Walls are going up. Nationalism and identity politics are on the rise once more. Tens of thousands of miles of fences and barriers have been erected in the past decade, and they are redefining our political landscape. There are many reasons why we erect walls: we are divided by wealth, race, religion, and politics. In Europe the ruptures of the past decade threaten not only European unity but, in some countries, liberal democracy itself. In China,...