The radioactive isotope U-235 can be “enriched” to higher levels of purity by the use of special centrifuges. Enriched to low levels (3.67 percent), U-235 can be used as fuel for nuclear power plants. Enriched to very high levels (90 percent), U-235 can become the basis for a nuclear weapon. Enrichment is a slow business in the early stages, but each successive step becomes much faster from higher levels of purity. According to one expert, it might take a month to enrich U-235 from 20 percent to 60 percent, then a week to go from 60 percent to 90 percent. However, more centrifuges are required to achieve each higher level of purity.
The development of nuclear material is one step. The development of the technology of making an actual weapon, and the development of ballistic missiles are additional steps. There is nothing to say that these steps have to be done sequentially, rather than in parallel.
Iran had developed a large infrastructure of uranium-enriching centrifuges, along with other elements of nuclear weapons development. Alarmed, the international community imposed increasingly severe economic sanctions on Iran. Eventually, the Iranian government agreed to negotiate.
The 2015 international agreement limited Iran to possessing 660 pounds of U-235 enriched to 3.67 percent and required the shut-down of many of its centrifuges. In return, Iran won removal of some—but not all—of the international economic sanctions. Many other issues regarding Iran’s foreign and military policy were set aside for further negotiations. Many economic sanctions were retained as leverage for these proposed future talks.
President Donald Trump soon abandoned the 2015 agreement and plastered Iran with sanctions. Iran then began moving away from compliance with the 2015 agreement. Iran increased its supply of U-235 that had been enriched to 3.67 percent; enriched some of its U-235 to 20 percent; restarted some its centrifuges; and blocked international inspectors from some of their agreed work. According to a February 2021 report from the International Atomic Energy Agency, Iran now possesses ten times the amount of enriched U-235 allowed under the agreement. If processed into weapons-grade material, that would be enough for three nuclear weapons. In addition, Iran has “largely ignored” an agreement on missiles and has allowed an agreement to expire that permits the security cameras to view Iran’s nuclear fuel.
There are several ways of interpreting the series of measures taken by Iran. One way is to see it as slicing the salami, seeing exactly what it can get away with without provoking an attack. Another way is to see it as a slow ratcheting up of pressure to both force a revival of the 2015 agreement and to improve Iran’s position in negotiations.
In the nature of the production process, holding down both the amount of enriched U-235 and the number of centrifuges are key. In mid-April 2021, Israel caused a major “mishap” at the centrifuge facility at Natanz. Perhaps several thousand centrifuges were destroyed.
 Rick Gladstone, William J. Broad, and Michael Crowley, “Iran Says It Won’t Make Bombs, But It May Be Inching Closer,” NYT, 18 April 2021.
 Thus it would take 500 centrifuges to move from 20 percent enrichment to 60 percent enrichment, and 600 centrifuges to move from 60 percent to 90 percent enrichment.
 As American bombing in Vietnam showed, this latter strategy doesn’t always work.
 David E. Sanger, “On Iran, Biden Walks a Tightrope Between Force and Diplomacy,” NYT, 29 June 2021.