Donald Trump in Fort Worth on Friday. (Mike Stone/Reuters)

AFTER SUPER Tuesday, Donald Trump could well be on the ballot come November — which means, among other things, that Americans are going to be offered a radical view of the world, and America’s role in it. The Trump foreign policy, at least as he’s described it in various outbursts on the campaign trail, is that the United States’ long-standing relationships and alliances are stacked against it, and that they must be downgraded, renegotiated or abandoned in favor of closer ties with, well, almost no one, except possibly Russian strongman Vladi­mir Putin. Quite apart from the ugly moral implications, the sudden, sullen U.S. retreat Mr. Trump advocates would be deeply destabilizing for an already unstable world.

The negative global impact would begin at the southern border. Relations between the United States and Mexico, once frosty but now warm thanks to a generation-long effort on both sides of the border, would be sacrificed to a pharaonic anti-migrant wall project and, possibly, a trade war, about which Mr. Trump says, “I don’t mind.” Mr. Trump would pick fights with Japan and South Korea over their purported insufficient payments for the U.S. troop presence in those countries, even though each country funds about half the non-personnel costs — and even though the costs of dealing with a U.S.-free Asia, in which Japan and South Korea might go nuclear for fear of China and North Korea, could be higher by far.

Meanwhile, Mr. Trump implies that he would forge cozy ties with strongmen and rely on them to keep order in their various domains and spheres of influence. His admiration for Mr. Putin — flattering himself, Mr. Trump boasts that the admiration is mutual — is already well-known. The revelation in the last Republican debate before Super Tuesday, though, was Mr. Trump’s benign view of Iraq’s Saddam Hussein and Libya’s Moammar Gaddafi: “At least they killed terrorists, all right?” he observed. This, regarding dictators with a long and proven record of backing murders of Americans, including 189 killed in the 1988 terrorist bombing of Pan Am Flight 103.

Mr. Trump leavened these geostrategic musings by noting it would be “a fantastic thing” to broker peace between Israelis and Palestinians, and merited a stance by him of “neutrality” toward the two parties. What he didn’t explain was how or why he would prove any more successful than past peacemakers, such as Secretary of State John F. Kerry, who recently spent a lot of time and U.S. prestige proving that the moment is not ripe.

To be sure, all of the above could change with the candidate’s mood, or with his naming of a foreign policy team — something Mr. Trump promised last September but has yet to deliver. For now, though, this is what we know: The likely Republican nominee has a wildly distorted assessment of the risks and rewards of U.S. engagement with the world. He envisions a country that seals itself off behind walls, physical and conceptual — and emerges only to bomb the odd terrorist camp or haul in a suspect for “much worse ” than waterboarding. Ultimately, this would prove far more costly to the American people than the institutions, alliances and commitments that Mr. Trump derides.