January 31, 2017 TIME
To give perspective on the widespread disapproval of Trump's executive orders, Peter Andreas, professor of international studies and political science, comments on the historical purposes of walls and the connotations it invokes. "The idea of erecting an almost 2,000-mile long wall between friendly, highly-integrated economies, is unprecedented, historically or today."
The history of walls is long. Here's where Donald Trump fits in (comments by Peter Andreas) – TIME