The globe has always been a world of walls, from the Great Wall of China to Hadrian's Wall to the Berlin Wall. But a new age of isolationism and economic nationalism is upon us, visible not just in Trump's obsession with building a wall on the Mexico border or in Britain's Brexit vote but in many other places as well. China has the Great Firewall, holding back Western culture. Europe's countries are walling themselves against immigrants, terrorism, and currency issues. South Africa has heavily gated communities, and massive walls or fences separate people in the Middle East, Korea, Sudan, India, and other places around the world.

In fact, at least 65 countries, more than a third of the world's nation-states, have barriers along their borders. There are many reasons why walls go up, because we are divided in many ways: wealth, race, religion, and politics, to name a few. Understanding what is behind these divisions is essential to understanding much of what's going on in the world today.

The Age of Walls examines how walls (which Marshall calls "monuments to the failure of politics"), borders, and barriers have been shaping our political landscape for hundreds of years, and especially since 2001, and how they figure in the diplomatic relations and geopolitical events of today.