WHAT WAS supposed to be a rare set-piece speech by Donald Trump on foreign policy Wednesday resembled a pastiche of his off-the-cuff postulations from the campaign trail, cobbled together under the slogan “America First.” Like the previous rhetoric, his proposals were loose, frequently contradictory and embedded in a bucket of falsehoods. Of these, the biggest was Mr. Trump’s claim that he could somehow reverse the historical tides that have created a globalized economy and remedy the complex security challenges of the 21st century with a simple “plan for victory with a capital V.”

It’s not clear whether Mr. Trump intended to associate himself with the original America Firsters, who campaigned to prevent U.S. entry into World War II and, in some cases, played down Nazi crimes and spun anti-Semitic conspiracy theories. But to the extent they had any coherence, his nostrums evoked the isolationism of Robert A. Taft or the mercantilism and fondness for authoritarian regimes of Pat Buchanan — Republicans who were resoundingly rejected by the party in the 1940s and 1990s, respectively. Among other things, Mr. Trump promised to “quickly” eliminate the U.S. trade deficit with China, force U.S. allies to pay for their own defense, and end “this horrible cycle of hostility” with Vladi­mir Putin’s Russia. He did this while excoriating President Obama as a leader who “picked fights with our oldest friends” and “bows to our enemies.”

Those weren’t the only contradictions. Mr. Trump said the United States had “no choice” but to abandon defense commitments to allies because of the allegedly weakened state of the U.S. economy. But minutes later the candidate was lamenting the shrinkage in the U.S. armed forces and declaring, “We will spend what we need to rebuild our military. It is the cheapest single investment we can make.” What’s more, he said, “your friends need to know that you will stick by the agreements that you have with them.”

“Fixing our relations with China is another important step,” said Mr. Trump. But his remedies were preposterous. He claimed that U.S. “economic power” over China could be used to force Beijing to “rein in” North Korea, “which is totally out of control” — a notion that would be laughed out of a high school foreign relations class. “We can both benefit or we can both go our separate ways,” he said of Sino-U.S. relations; we pity the China analysts charged with making sense of that.

Mr. Trump blamed previous administrations for making a mess of the Middle East — a reasonable claim, but one he littered with false assertions. He again claimed, against the known record, to have opposed the Iraq War well before it began. He said, falsely, that the Islamic State was exporting oil from Libya. Then there were the flagrant lies that “there are scores of recent migrants inside our borders charged with terrorism” and that “for every case known to the public, there are dozens and dozens more.”

One line in Mr. Trump’s speech did have the ring of truth. Having elsewhere stressed the need for consistency and reliability in U.S. foreign policy, he blurted: “We must as a nation be more unpredictable.” It’s a good bet that the United States under a President Trump would be just that — to the peril of itself, and the rest of the world.