In the days after President Donald Trump signed an executive orderauthorizing "immediate" construction of a wall along the U.S.-Mexico border, followed by another executive order temporarily prohibiting refugees and citizens of seven predominantly Muslim countries from entering the U.S. (and blocking Syrian refugees indefinitely), four words were heard in many places across the United States this weekend: "No Ban! No Wall!"

As thousands of protesters camped out at JFK International Airport in NYC and other international airports nationwide to protest Trump's actions, the sentiment became a popular one for signs and chants.

But Trump's wall is not just controversial. A look at which groups have put up walls throughout history—and why—can help us understand just how unique the proposed wall would be.

"The distinctive thing is this is a wall against immigration—and to some degree also contraband drugs and gun-running—whereas historically, the other famous or infamous walls have almost always been about blocking invading armies," says Wendy Brown, author of Walled States, Waning Sovereignty and professor of political science at the University of California, Berkeley.