https://responsiblestatecraft.org/iran-nuclear-deal/
To fully understand the enormity of Trump’s decision to leave the Iran deal, consider this: When the U.S. and Iran were complying with the deal, it was estimated that it would take Iran about one year to produce enough fissile material (in this case, weapons grade uranium) for a nuclear bomb (known as the “breakout” time). The states negotiating with Iran (the United States, Russia, China, Great Britain, France, and Germany) assessed that this would be enough time to respond to possible violations and prevent Iran from producing a bomb. Even if Iran were to acquire sufficient fissile material, it could still take another year for Iran to make a deliverable nuclear weapon. As of May, 2018, the deal was working and considered (by most) to be a great success.
Then President Trump unilaterally left the deal, calling it a “horrible one-sided deal that should have never, ever been made.” And now we are in a much worse place. Iran says it has no intent to produce nuclear weapons and U.S. intelligence sees no current efforts by Tehran to weaponize, yet Tehran is believed to be not one year but just weeks from being able to produce enough fissile material for a bomb if it chooses to do so.