Vice President Biden’s promise to resume the Obama administration’s appeasement of the theocratic dictators of Iran is a shocking marker. It is the headline news in the foreign policy platform Mr. Biden just outlined in a speech at New York University. It comes from the politician said to be the most centrist figure in the Democratic scrum. So it will reinforce President Trump’s credibility in the coming campaign.

On the one hand, this isn’t surprising. Mr. Biden himself has always been a weak figure in the Middle East. When he was on the Foreign Relations Committee, he was so condescending to Menachem Begin that the visiting Israeli premier publicly rebuked him in our own Capitol. As Vice President, Mr. Biden ruined a visit to Israel by throwing a tantrum over a ruling by a Jerusalem zoning body.

Mr. Biden failed to attend the speech to a joint meeting of Congress by Prime Minister Netanyahu of Israel (it was boycotted by Secretary Clinton’s running mate, Senator Kaine). Most significantly, Mr. Biden sided with Secretary of State Kerry and Europe on the articles of appeasement with Iran, rather than back up the Senate Mr. Biden constitutionally served as the equivalent of speaker.

One thing to remember about the Iran deal that Mr. Biden now promises to rejoin: It was opposed — “overwhelmingly,” said the New York Times — by both houses of Congress at the time the deal was struck. Yet President Obama and Secretary Kerry, without a peep of opposition from Mr. Biden, took the deal to the United Nations and voted for it there in opposition to our own Congress.

The aim was to trap America in the deal despite Congress. So, naturally, it was one of the premier planks in President Trump’s 2016 platform. It is one of the reasons millions supported him (and one of the reasons the Sun endorsed Mr. Trump). Americans were horrified when they discovered that, as part of the deal, the Obama administration had airlifted pallets of cash — billions — to Iran.

No one wants a full-blown war with Iran. We’re already, though, in a war with it now, in the Middle East theater and beyond. It may be a relatively low-grade, undeclared war, involving mainly proxies and terror. No one, though, wants it to get out of hand, as Mr. Trump demonstrated when he called off an attack over enemy actions in the Gulf of Oman. He may be a lot of things, but trigger happy he isn’t.

The surest way for the Iran conflict to get out of hand, in any event, is to pursue the course of appeasement that Mr. Biden promises. And to align ourselves with a Europe that has wavered over appeasement since World War II. It did that over the deployment of cruise and Pershing missiles, until Reagan made it clear we would stand. Its waffling over Iran is a classic case.

Mr. Biden issued a campaign video mocking President Trump’s approach to foreign affairs, his embrace of North Korea’s dictator, his credulity in respect of the Saudis after the killing of Jamal Khashoggi, his flattery of President Putin. Mr. Biden suggests America is to blame for the advance of the Iranians. The thing to remember is that their breakout followed a campaign of appeasement of which Mr. Biden was a part.