In July 2015, the Obama Administration signed an agreement limiting Iran's nuclear weapons and ensuring enhanced monitoring in exchange for Iran's relief from sanctions. This followed six years of careful negotiations involving the five permanent United Nations Security Council members plus Germany, the European Union and Iran. Before the agreement, Iran was less than one year away from having nuclear weapons. The accord extended this "safe time" to 10 years, and imposed intrusive inspection to prevent cheating.
In May 2018, President Trump pulled the U.S. out of this treaty and re-established tough sanctions on Iran despite having no evidence of Iranian cheating and despite the International Atomic Energy Agency's certification that Iran had abided by the agreement.
Why, then, did Trump withdraw ? He provided two reasons: The temporary nature of the restrictions, and the treaty's failure to also cover Iran's non-nuclear aggressive activities (ballistic missiles and support for militant groups). Another reason: Trump's alliance with Israeli leader Benjamin Netanyahu, who boasted he was responsible for convincing Trump to renounce the deal.
Regarding Trump's two stated reasons: Parts of the accord had a 10- to 15-year time limit. But this was enormously safer than the pre-treaty situation and there was every prospect of extending the treaty's time-frame provided Iran was treated with respect and permitted to develop its economy free of sanctions. Iran has continued to arm itself with ballistic missiles while supporting terrorism, but so have other Mideast nations such as U.S. ally Saudi Arabia, which provided manpower and weapons to Sunni terrorists throughout Syria's tragic civil war to overthrow the Assad regime. It's always a mistake to complicate nuclear weapons negotiations -- an existential issue for the planet -- with such non-nuclear military concerns.
The 2015 treaty was a very good deal for all sides, and an outstanding achievement of the Obama administration. As to why Trump renounced it: Former CIA director Michael Haydon analyzed the information behind Trump's decision and concluded that the only plausible explanation is Trump's unreasoning hostility to everything Obama did.
The accord limited Iran in five ways:
• Uranium enrichment was limited to 4 percent. Natural uranium contains two types of uranium atoms: those that will "fission" (explode) and those that won't. Fissionable atoms form less than 1 percent of the total. "Enrichment" is the process of increasing this percentage. Enrichment to 90 percent is required for bomb material, so 4 percent is far from bomb-grade. The agreement reduced Iran's stockpile of 4-percent uranium to only 300 pounds, far too little for a bomb. Since Trump's rejection of the treaty, Iran has produced 5,400 pounds of low-enriched uranium, enough to enrich further and make two bombs.
• It eliminated Iran's stockpile of 20-percent-enriched uranium that Iran claimed was reserved for medical applications. Because enrichment is an "exponential" process, 20 percent is a short step away from bomb-grade.
• It reduced Iran's permitted centrifuges by two-thirds. Centrifuges are long vertical tubes that act like cream separators: When filled with uranium gas and spun around their central axis, the lighter fissionable atoms drift toward the axis where they can be captured for "use" (i.e. for bombs).
• It limited enrichment to one plant (at Natanz, not the heavily fortified Fordow plant) using older-style less-efficient centrifuges.
• And it required Iran to eliminate its "heavy water reactor" and install no new reactors of this type. Such reactors can produce bomb-grade plutonium, another fuel for nuclear weapons (India got its first bomb this way).
Thanks to Trump's withdrawal, Iran of course felt free to ignore the agreement. It has now produced 5,400 pounds of 4 percent uranium, vastly exceeding the 300-pound limit. It has increased its number of gas centrifuges and installed highly efficient centrifuges, far beyond the treaty's limitations. Following the recent destruction (probably by Israel) of Iran's Natanz enrichment plant, Iran is rebuilding it inside a mountain. They have redesigned the Arak reactor and returned it to operations. Today, Iran's breakout time to build a bomb is only three months. Prior to Trump's decision, the minimum breakout time was 12 months because Iran had only a very small stockpile of 4 percent uranium.
Thus Trump has managed to reduce U.S. security, increase the threat of nuclear war, increase the probability that Iran will race for a nuclear weapon, and increase the threat of general war with Iran. With "friends" like Trump, America doesn't need enemies.
That's the bad news. The good news is that President-elect Biden stated, before this year's election, that he would return to the nuclear deal with Iran.