The treaty, obscurely named the Joint Comprehensive Plan of Action (JCPOA), was a deal in which Iran put its nuclear programme on hold in return for all the major powers easing sanctions on the country.
The treaty was a major accomplishment of Barack Obama’s presidency, which may be why Trump pulled the United States out three years later.
Iran had been scrupulously obeying the terms of the JCPOA, and none of the other signatories (China, France, Germany, Russia, and the United Kingdom) had any complaint about its behaviour.
Trump, however, clamped strict trade sanctions back on Iran, which naturally responded by resuming its work on enriching uranium.
The Iranians claim that they are only enriching uranium for nuclear fuel and other peaceful uses, but nobody believes them.
The experts say they must want at least a "nuclear threshold" capability, because nobody enriches nuclear fuel to 60% uranium just to fuel power plants. The Iranians want at least the ability to build their own nukes fast in an emergency.
What kind of emergency? Well, the Iranians know that there are Israeli and American nuclear weapons targeted on them, and currently they have no nuclear weapons of their own.
Getting them would no more prove that they plan to use them than it did when the Soviet Union, Britain, France, China, Israel, India, Pakistan and North Korea got them.
However, many other nuclear states see Iran as less eligible to have nuclear weapons because it is ruled by an elderly and fanatical religious elite that regularly issues blood-curdling threats to its numerous enemies.
That was why they banded together to back the JCPOA treaty, but Trump trashed that and here we are.
Iran has used the past seven years to work away on its uranium enrichment project, first at a fairly leisurely pace but then more quickly as the prospect of another Trump regime loomed. Now they are almost there.
The International Atomic Energy Agency said last month that Iran had now stockpiled almost 275kg of uranium enriched to 60% purity, just short of weapons grade. The last step, to 90% pure, is the easiest, and could be done in a few days or, at most, a few weeks.
That is enough fissile material for six nuclear bombs, although actually fabricating crude but effective nuclear bombs would probably take a further six months. If they want to put nuclear warheads on missiles, they could be looking at as much as two years.
Given how many Iranian missiles were intercepted or missed their targets in the past two rounds of fighting, however, Teheran might prefer not to put its precious warheads-to-be in the nose-cones of ballistic or cruise missiles.
Trucks are not a good bet for delivering across Israel’s border either. Ships? Who knows?
None of this speculation means that Iran is planning to use nuclear weapons on Israel (or anywhere else) as soon as it gets them.
It does mean that potential target countries will be thinking that it might, and making their own plans to pre-empt that contingency if they can. It is definitely getting more dangerous.
None of this would be happening if Trump had just left the JCPOA alone. Iran would still be years and years away from nuclear weapons, and even Trump might settle for just negotiating another 10-year term for the treaty.
But maybe not. Trump is very much under the influence of Israel’s Prime Minister Benjamin Netanyahu, who really does want the US to attack Iran. (He will even help.)
To make matters worse, the Iranian regime feels that it has its back to the wall.
Nationwide outbreaks of protest against the government of the ayatollahs are coming closer and closer together. The sanctions have crippled the economy and everybody is feeling the pinch.
All of Iran’s allies in the Arab world — Hizbollah in Lebanon, Hamas in Gaza, the Assad regime in Syria, even the Houthis in Yemen — are being stripped away.
And Trump seems to think that he can retrieve his original blunder (not that he admits it was a blunder) by writing a letter to Grand Ayatollah Ali Khamenei.
He gave the Supreme Leader two months to negotiate a new nuclear deal or the US and Israel will strike Iran’s nuclear facilities.
This is typical Trump stuff: set very tight deadlines, demand unrealistic terms and threaten "hell" if the deadline is not met.
It generally does not work well, especially when you are dealing with fanatics. We have the makings of a really nasty war here, possible including a local nuclear war or (if Iran’s nukes are still not ready) at least a dirty bomb or two.
• Gwynne Dyer is an independent London journalist.
