Another reason to take it seriously: The plan is consistent with Trump’s extractive, neo-mercantilist worldview. In a recent interview with The New York Times, he characterized every major U.S. relationship as a zero-sum transaction from which America could wring more value. As president, Trump would slash U.S. funding for NATO and the United Nations; pressure Japan and South Korea to pay more for hosting U.S. troops and perhaps even to develop their own nuclear weapons so they’ll quit relying on America’s nuclear deterrent; make Germany and the Gulf states provide the money for “safe zones” in Syria; and coerce Saudi Arabia into “reimbursing” the United States for protecting the kingdom.

It’s worth pausing to appreciate the significance of this worldview. Since the end of the Cold War, U.S. presidents have wrestled with a novel question: What does it mean to be the world’s sole superpower? In broad strokes: Bill Clinton cast the United States as a singular superpower—an “indispensable nation,” charged with mobilizing the international community to “advance peace and freedom and democracy” around the world. George W. Bush unilateralized and militarized and evangelized Clinton’s paradigm. Barack Obama has chafed at the “free riders,” overextension, and hubris that indispensability breeds. He’s striven to make the U.S. more of a self-aware superpower—cognizant of its limits and strengths and weaknesses.

Trump’s logic begins in a similar place as Obama’s, but then marches off in a different direction. He argues that American indispensability is a rotten deal that has turned a once-respected economic powerhouse into a “poor” nation burdened by $19 trillion in debt, lacking real friends but flush with freeloaders. He wants the U.S. to be a self-interested superpower, conducting diplomacy primarily through protectionist trade policies. And to “make America great again,” he’s willing to rethink and upend the key components of indispensability: America’s intricate alliances, overseas military bases, and mutual defense pacts, plus the pillars of the post-World War II international system—NATO, the UN, free trade, the global nuclear nonproliferation regime—that the United States helped design.

“I’m not isolationist, but I am ‘America First,’” Trump told The New York Times, using a term for his foreign-policy doctrine that David Sanger of the Times had suggested. “We have been disrespected, mocked, and ripped off for many, many years by people that were smarter, shrewder, tougher.” It’s a theme Trump has trumpeted for decades. “Make Japan, Saudi Arabia, and others pay for the protection we extend as allies,” he wrote in 1987. “Let’s not let our great country be laughed at anymore.”

Which brings us back to the wall. How do you get “others” to pay? The answer, when it comes to Mexico, offers a case study in what Trump’s America-First theory might look like in practice.