All nations have borders, but only the United States has had a frontier — or at least a frontier that served as a symbol for freedom, synonymous with the possibilities and promises of modern life and held out as a model for the rest of the world to emulate.

For over a century, the American frontier represented the universalism of the nation’s ideals. It suggested not only that the country was moving forward, but also that the brutality involved in moving forward would be transformed into something noble. Extend the sphere of America’s influence, as James Madison believed, and you would ensure peace, protect individual liberty and dilute factionalism. As our boundaries widened, all of humanity would become our country. There was no problem caused by expansion that couldn’t be solved by more expansion.

But today the frontier is closed. The country has lived past the end of that myth. After centuries of pushing forward across the frontier — first, the landed frontier, then the frontiers of expanding economic markets and sweeping military dominance — all the things that expansion was supposed to preserve have been destroyed, and all the things it was meant to destroy have been preserved. Instead of peace, there is endless war. Instead of prosperity we have intractable inequality. Instead of a critical, resilient and open-minded citizenry, a conspiratorial nihilism, rejecting reason and dreading change, has taken hold.

Where the frontier once symbolized perennial rebirth, Donald Trump’s border wall — even if it remains mostly phantasmagorical, a perpetual negotiating chip between Congress and the White House — now looms like a tombstone.