Donald Trump won the White House twice on a promise to close the border. Now he waxes poetic about reopening the frontier — whose “spirit,” he said yesterday in his second Inaugural Address, “is written into our hearts.” This month, he talked about buying Greenland from Denmark, annexing Canada, retaking the Panama Canal and renaming the Gulf of Mexico the Gulf of America. “What a beautiful name,” Mr. Trump said, pronouncing the phrase with a decided stress on its last syllable: A-mer-i-CA, not A-MER-i-ca.

This expansionist turn is surprising for a politician best known for wanting the nation to hunker down behind a border wall. But Mr. Trump is smart. He knows, it seems, that the angry, inward-looking nationalism that first won him office can be self-destructive, as it was during his besieged first term. These calls, then — to make America not just great but also greater in size — tap into a more invigorating strain of patriotism: a vision of a United States that is forever growing, forever moving outward.

Mr. Trump’s recent remarks have electrified his base, with MAGA enthusiasts using social media to circulate battle plans to seize Canada and maps of a United States that stretches from the Arctic to Panama. But Mr. Trump is also harking back to the founders, many of whom similarly thought the United States had to expand to thrive. “Extend the sphere,” wrote James Madison in 1787; increase the “extent of territory,” and you’ll diffuse political extremism and stave off class warfare. “The larger our association,” said Thomas Jefferson in 1805, speaking of his Louisiana Purchase, “the less will it be shaken by local passions.”

In the years that followed, the United States moved across the continent with dizzying speed, citing the doctrine of conquest as it took Indian and Mexican land, reaching the Pacific and then seizing Hawaii, Puerto Rico and other islands.

And later, in the 20th century, even after the United States, along with much of the world, renounced the doctrine of conquest, our leaders still conjured up a sense of potentially limitlessness expansion, in the opening of markets for U.S. exports, in wars to rid the world of evils, in upward mobility and a growing middle class and in science and technology, which offered what the historian Frederick Jackson Turner once said the American West promised: “perennial rebirth.”

Mr. Trump is tapping into this social and intellectual history, promising to “pursue our Manifest Destiny into the stars” — even “to Mars.” But he does so in that witchy style he has perfected, which makes conventional ideas sound outlandish.