President Trump’s promise to build a wall along the U.S.-Mexico border was a defining and divisive talking point throughout the 2016 campaign, rallying his conservative base enough to help win him the election. In trying to fulfill one of his most popular campaign promises, President Trump has halted the U.S. government in the longest federal shutdown in the country’s history, demanding more than $5 billion in funding for a wall at the U.S.-Mexico border.

But experts point out to Rolling Stone that border walls have failed time and again throughout history — whether it’s the fall of the Berlin Wall or the penetration of France’s Maginot Line in World War II — and they’ve even failed at the United States’ southern border, where the Clinton administration began constructing barrier fences in the early 1990s.

So, why have governments continued to fall back on the idea of erecting such physical barriers? Some of the world’s leading border experts say the reasons range from a rise in nationalism in response to globalization, a dramatic increase in population, racism, and the looming threat of terrorism in a post-9/11 world. The common thread being fear.