This week marks the first stage of President Trump’s attempt at dealing a final, fatal blow to President Obama’s Iran deal. America’s United Nations team expects a Security Council vote on a draft resolution mandating an indefinite extension of a global ban on Iran’s ability to legally buy and sell weapons.

A previous council resolution — the only legal document backing Mr. Obama’s nuclear deal with Iran — contains several “sunset” clauses phasing out Iran sanctions. Barring an extension, one of those clauses would allow Iran, by October, to buy aircrafts, missiles and other arms, and export them to proxies.

America’s resolution is widely expected to tank, as Russia and China threaten to veto any extension. Our UN team may also fail to gather nine necessary supporters among the council’s 15 members, in which case vetoes will not even be necessary. “The US text goes beyond a simple extension of the arms embargo,” gripes one Western diplomat.

Fearing the unraveling of the entire Iran deal, Britain, France and Germany offered a compromise, temporarily extending the embargo beyond October. America revised its own text as well Tuesday, calling for an embargo extension “until the council decides otherwise,” but China and Russia, eager to resume legal arms sales to Iran, are widely expected to reject such compromise.

American officials urge allied council members to ignore Tehran’s threats. They say those include tacit warnings that unless the arms ban ends, the Islamic Republic’s behavior will take a turn for the worse.

“When you play by the house rules, the house always wins,” a former State Department official, Brian Hook, said last week. “We’re not going to play by Iranian rules.” One way or another, Mr. Hook added in a phone call with reporters, America will extend the arms ban.

Shortly after that briefing, conducted alongside America’s UN ambassador Kelly Craft, Washington announced Mr. Hook would step down as State’s Iran point man. He was replaced by a veteran diplomat, Elliott Abrams, who is as hawkish on Iran as Mr. Hook.

Considering the persistent council resistance, how could America maintain the global arms embargo? United Nations watchers reckon this week’s proposed resolution is only America’s initial salvo, leading up to “snapback.” That is the mechanism the Obama team built into the 2015 JCPOA-endorsing resolution in an attempt to convince skeptics of the deal that America could opt out of it at will.

In theory, the snapback mechanism provides a fast way to kill the Iran deal. As written into a council resolution, though, it’s a complex parliamentary maneuver that allows each of the JCPOA parties — Russia, China, France, Britain, Germany and America — to declare Iran in non compliance and reimpose all previous sanctions. Iran, too, can opt out.

Russia is threatening to challenge America’s right to invoke snapback, arguing President Trump has formally walked out of the deal, while American legal eagles rely on a plain reading of the 2015 resolution’s text.

Either way, diplomats predict a major Security Council showdown, leading to an impasse that can unravel much of the delicate balance among the members, rendering the UN’s most consequential body dysfunctional.

This is a moment to remember what this fight is all about. As Ambassador Craft put it in a Fox News interview Monday, “the choice is between the number one state sponsor of terrorism around the world, or peace and security.”

Is Ms. Craft any different from a predecessor, Samantha Power, who in 2016 facilitated a council resolution declaring Jewish presence in the West Bank and Jerusalem illegal? That resolution forced the next president, Mr. Trump, to contend with a UN-dictated policy he publicly opposed on the campaign trail.

Would candidate Joseph Biden face a similar challenge if the UN erases the Iran deal he favored as vice president?

There are major differences: Ms. Power’s 2016 resolution, a novel attempt at peacemaking, was enacted in December, after Mr. Trump had already been elected president. The timing of the current showdown is dictated by a previous resolution’s calendar that ends the arms embargo in October. Plus, while the polls suggest Mr. Biden is ahead at this date, it’s no lead pipe cinch that Mr. Biden will prevail in November.

More crucially, the Obama-era resolution was born out of the White House’s animosity to a democratically elected prime minister of America’s closest Mideast ally. The current one is an attempt at course correction on Washington’s deal with a tyrannical regime that fancies itself America’s most formidable foe.

Europeans cling to the Obama-era deal, but as yet it has failed to curb the Islamic Republic’s thirst for regional dominance, or even achieve its top goal — temporarily slowing down Tehran’s nuclear aspirations.

Mr. Biden won’t admit it, but if he wins in November he will be better off starting from scratch, unsaddled with the existing, failed Iran deal.

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Twitter @bennyavni