Iran Breaches Critical Limit on Nuclear Fuel Under 2015 Deal

But a section of the pact allows the Europeans to invoke a so-called snapback of sanctions if Iran violates the terms. The Iranians say they are within their rights because the reimposition of sanctions last year by the United States gave them the grounds to halt their commitments, as well.

For advocates of the 2015 deal, like former members of the Obama administration, Mr. Trump pushed Iran into the announcement. Among the recently imposed sanctions was one that threatened action against any country that bought low-enriched uranium from Tehran. To comply with the stockpile limits, Iran shipped low-enriched uranium to Russia in return for natural uranium. With that exchange now barred, it was only a matter of time before Iran exceeded the limits.

Even before the announcement, the Pentagon and the nation’s intelligence agencies — led by the C.I.A. and the National Security Agency — were beginning to review what steps to take if the president determined that Iran was getting too close to producing a bomb.

A decade ago, the Obama administration conducted a highly classified cyberattack, code-named Olympic Games, at the Natanz enrichment site. The breach neutralized Iran’s centrifuges, which spin at supersonic speeds to enrich uranium, and destroyed about 1,000 of the 5,000 machines then in operation. But after two years, Iran rebounded, and when the nuclear accord came into effect, it had more than 17,000 centrifuges, most of which were dismantled under the agreement.

If the United States targets Iran’s uranium enrichment operations, experts say, it is likely to take aim again at the Natanz site. But this time, the Iranians appear far better prepared. Other major nuclear sites, including the primary production facility for converting raw uranium to a gas form, and factories that produce next-generation centrifuges, are also likely targets, according to former officials.

In the weeks before the announcement, Saudi Arabia’s state news media has called for “surgical strikes” against Iran, as has Senator Tom Cotton, who pressed for military action after the downing of the drone. Mr. Trump initially agreed, then pulled back.