In many ways, the barrier-building is being driven by fear.
Most of the new walls are being erected within the European Union, which until recently was nearly borderless. Britain is going further, rolling up its bridges to the continent by voting to exit the E.U. Intended to counter migrants and terrorist attacks, these moves are not limited to Europe. In the Middle East, Tunisia is erecting a desert barrier with lawless Libya to insulate itself from unrest and an Islamic State-led insurgency.
In Asia, India and Burma are encircling Bangladesh with hundreds of miles of razor wire to block migrants and counter religious extremism.