Jeremy Corbyn leaves the Labour Party’s headquarters in London on Sept. 14. (Justin Tallis/Agence France-Presse via Getty Images)

SOME IN the United States see Jeremy Corbyn, the newly elected leader of Britain’s Labour Party, as an analogue of Bernie Sanders, the surging socialist in the Democratic presidential primary. Mr. Sanders himself said he was “delighted” by Mr. Corbyn’s win. Yet what the Guardian newspaper called “the most astonishing leadership victory in any major British political party in modern times” was not merely a blow against “mass income and wealth inequality,” as Mr. Sanders described it. It also validated a radically anti-American agenda that could accentuate Britain’s drift away from the trans-Atlantic partnership.

Mr. Corbyn espouses a foreign policy whose guiding principle is to oppose the United States and Israel by all means. It has led him to label as “friends” such disparate political forces as Hamas, Hezbollah and the populist government of Venezuela and to accept funding from organizations designated by the U.S. government as terrorist groups. Mr. Corbyn endorsed the Iraqi insurgents who fought U.S. troops and equated the Islamic State’s overrunning of Iraqi cities with the 2004 U.S. offensive in Fallujah. He said that Washington, rather than Moscow, is to blame for the civil war in Ukraine. In an interview with Iran’s state television channel, he called the U.S. raid that killed Osama bin Laden a “tragedy.”

A member of Parliament since 1983, the 66-year-old Mr. Corbyn unabashedly espouses the agenda defended by the Labour Party three decades ago. He would nationalize railroads, utilities and all other public services, massively increase government spending, and even reopen the coal mines closed by Margaret Thatcher; global warming was not an issue in the 1980s. Like Labour’s leaders then, he favors British withdrawal from NATO and perhaps the European Union, and the unilateral scrapping of U.K. nuclear weapons.

Thanks to those stands, Labour lost four consecutive national elections before moving back to the center under Tony Blair — whom Mr. Corbyn favors prosecuting for “war crime[s].” His supporters say he was propelled into power by a tide of new, younger voters fed up with traditional politics. It’s easy to exaggerate the size of that wave: Mr. Corbyn, who benefited from a new Labour Party rule allowing anyone who paid the equivalent of $5 to vote in its primary, collected a quarter-million votes, or about half of 1 percent of the British electorate. Most British analysts believe he will struggle to survive as Labour’s leader until the next election, due by 2020, and that if he does Labour will lose in a landslide.

British politics, however, looks less than predictable in an era where the Scottish National Party is the third largest in Parliament and the ruling Conservative Party is riven by internal disputes. The austerity forced by the Great Recession and a backlash against the wars in Iraq and Afghanistan have already prompted a significant retrenchment by the government of David Cameron from the country’s global role. Mr. Corbyn can be expected to lead a fierce resistance to Mr. Cameron’s incipient effort to step up Britain’s contribution to the fight against the Islamic State. He will do his best to sabotage relations with Washington. Those who favor the strategic partnership between Britain and the United States can only hope that Labour’s new leader is not the voice of an emerging generation but the last gasp of an old one.