The U.S. and Europe are still begging Iran to return to the increasingly irrelevant 2015 nuclear deal. But a new report on Iranian nuclear advances shows how far Tehran’s program has come—and why President Biden should rethink his strategy.

Donald Trump withdrew from the 2015 Joint Comprehensive Plan of Action, the nuclear accord’s formal name, in 2018 and began his “maximum pressure” sanctions campaign. The economic restrictions weakened the regime and gave the U.S. more leverage. Yet Tehran waited out Mr. Trump, betting...

A number of new generation Iranian centrifuges are seen on display during Iran's National Nuclear Energy Day in Tehran, April 10.

Photo: iranian presidency office/WANA/Reuters

The U.S. and Europe are still begging Iran to return to the increasingly irrelevant 2015 nuclear deal. But a new report on Iranian nuclear advances shows how far Tehran’s program has come—and why President Biden should rethink his strategy.

Donald Trump withdrew from the 2015 Joint Comprehensive Plan of Action, the nuclear accord’s formal name, in 2018 and began his “maximum pressure” sanctions campaign. The economic restrictions weakened the regime and gave the U.S. more leverage. Yet Tehran waited out Mr. Trump, betting that he would lose the election, as he did.

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Meanwhile, Tehran activated advanced centrifuges, while stonewalling international nuclear inspectors, enriching uranium at higher concentrations and stockpiling more of it. Perhaps most troubling is how much the “breakout” time to a nuclear weapon has shrunk thanks to Iran’s better understanding of advanced centrifuges, which produce enriched uranium more efficiently.

“Unless compensatory steps are taken, such as destroying rather than mothballing advanced centrifuges, a renewed [nuclear deal] will not maintain a 12-month breakout timeline to produce enough weapon-grade uranium for a nuclear weapon,” David Albright and his colleagues at the Institute for Science and International Security write in a Dec. 2 report. “If Iran mothballs its advanced centrifuges, timelines of only five to six months are likely.”

And these are conservative estimates: “Because of the risk that Iran has accumulated a stock of undeclared assembled centrifuges as well as sensitive centrifuge components, breakout timelines could be further reduced.” The right stockpile of enriched uranium and advanced centrifuges could “be enough to breakout at a clandestine enrichment site and produce enough weapon-grade uranium for a nuclear explosive in about one month.”

The Biden crowd blames all this on Mr. Trump, but the ease and speed with which Iran deployed its advanced centrifuge capacity shows the flaw in the accord that he left. The original deal left Iran with the ability to restart everything on a dime, and with limited International Atomic Energy Agency (IAEA) ability to monitor its progress. It did buy some time on nuclear issues—but at the cost of strengthening Iran with money to promote jihadist militias and terrorism. Paying Tehran billions in sanctions relief, much of which was used to destabilize the region, wasn’t worth the limited nuclear restrictions.

Iranian negotiators took maximalist positions at the latest round of Vienna talks, demanding heavy sanctions relief in exchange for minimal commitments. The IAEA recently said Iran had begun enriching uranium with advanced centrifuges at its Fordow nuclear facility. Under the 2015 deal’s terms no enrichment is allowed at Fordow, but Tehran seems to have calculated that such provocations increase its leverage and may coax more concessions from the U.S.

The White House knows how perilous Iran’s advances are, and so does Israel. The difference is that Washington is humoring this nuclear blackmail as Jerusalem weighs whether pre-emptive action, military or otherwise, is needed to rein in the Iranian nuclear program.

Mr. Biden’s negotiators should wonder what gave Tehran the confidence to keep escalating. Rather than hope for an Iranian change of heart, Team Biden should prepare for how to stop the rogue nation from obtaining the bomb if negotiations fail.

Joe Biden came to office promising he'd take a tough stance with Vladimir Putin, but his foreign policy decisions to date haven't deterred Russia amassing thousands of troops in readiness to invade Ukraine. Images: Getty Images/Maxar Composite: Mark Kelly The Wall Street Journal Interactive Edition