The strike on three of Iran’s nuclear facilities by the United States was unwise and unnecessary. Now that it’s done, I very much hope it succeeded.

That’s the paradox for many former officials like me who worked on the Iran nuclear problem during previous administrations. We shared a determination that Iran never be allowed to produce or possess a nuclear weapon. Iran without a nuclear weapon is bad enough: a leading state sponsor of terrorism; a destructive and destabilizing force via its proxies in Lebanon, Syria, Gaza, Yemen and Iraq; an existential threat to Israel. An Iran with a nuclear weapon would feel emboldened to act with even greater impunity in each of those arenas.

So why was the strike a mistake?

First, it never should have come to this. In 2015, the Obama administration, together with Britain, France, Germany, Russia, China and the European Union, reached agreement with Tehran on the Joint Comprehensive Plan of Action, or J.C.P.O.A. The nuclear deal effectively put Iran’s program to make fissile material, the fuel for a nuclear weapon, in a lockbox, with stringent procedures for monitoring Iran’s nuclear program. The deal pushed “breakout” — the amount of time it would take Iran to produce enough weapons-grade uranium for a single nuclear weapon — to at least one year. If Iran reneged on the agreement or refused to extend it when certain provisions expired after 15 years, we would know it and have plenty of time to respond, including, if necessary, militarily.

In 2018, President Trump tore up the agreement and replaced it with … nothing. In response, Iran accelerated its enrichment, quite likely reducing its breakout time to a matter of days or weeks. Mr. Trump, in essence, is now trying to put out a fire on which he poured gasoline.

Second, fissile material is a necessary but insufficient element for a bomb. You also need an explosive weapon. As of now — and there are conflicting messages coming from within the Trump administration — our intelligence agencies believe Iran has not yet made a decision to weaponize. If and when it does, it would take Tehran 18 to 24 months to produce an explosive device, according to some estimates. In other words, there was still time for diplomacy to work, and the situation wasn’t nearly the emergency that Mr. Trump portrayed it to be.

Third, experts I’ve spoken to had real doubts about the ability of the Massive Ordnance Penetrator, or M.O.P. — the 30,000-pound bombs unique to America’s arsenal that were dropped on Iran’s nuclear sites — to fully incapacitate the Fordo site and other deeply buried or fortified components of Iran’s nuclear program. Initial reports suggest that while Iran’s nuclear infrastructure was severely damaged, it was not destroyed.