U.S. President Donald Trump today announced that he is pulling the United States out of a ۲۰۱۵ agreement with Iran and other nations to limit Iran’s nuclear program .

“At the heart of the Iran deal was a giant fiction,” Trump said, that “a murderous regime” could have a civilian nuclear program without pursuing nuclear weapons.

The Iran deal was intended to slow and delay Iran’s efforts to build a nuclear weapon. It lifted economic sanctions on Iran in exchange for a number of moves to shut down its uranium enrichment efforts and related programs.

“The fact is that this was a horrible, one-sided deal that should have never been made,” Trump said in remarks at the White House announcing that he will restore economic sanctions on Iran. He argued that international inspections are inadequate to monitor Iran’s compliance with the deal—a claim disputed by many experts—and that it did not address Iran’s efforts to produce ballistic missiles or its support for groups that the United States has labeled as terrorist organizations.

In a statement, the White House said the “re-imposed sanctions will target critical sectors of Iran’s economy, such as its energy, petrochemical, and financial sectors. Those doing business in Iran will be provided a period of time to allow them to wind down operations in or business involving Iran.”

Trump said his administration will pursue new agreements with Iran, but provided no details.

How fast could Iran restart its nuclear weapons program?

It depends on how aggressively it challenges the status quo, which includes extensive independent monitoring of its nuclear facilities by the International Atomic Energy Agency (IAEA) . Iran has officially denied it has a military program, but prior to the ۲۰۱۵ agreement, intelligence analysts estimated the nation would need months to enrich enough uranium to produce a weapon , and at least a year to develop a warhead and missile capable of carrying it to a target. Now, Iran’s government claims it could quickly restart its nuclear program if the deal collapses, but it is not clear how much time that restart might add to its efforts to create a weapon.

One task would be rebuilding and restarting thousands of centrifuges used to enrich uranium to weapons-grade levels. According to IAEA, Iran has complied with the agreement’s requirements to dismantle or mothball nearly ۱۳,۰۰۰ centrifuges at two facilities—at Fordow and Natanz—and eliminate or ship to other countries ۹۵% of its existing stockpile of enriched uranium. Some ۵۰۰۰ first-generation centrifuges continue to operate at the Natanz enrichment facility, along with ۱۰۰۰ at the Fordow facility. The agreement also places a cap on the amount of low-enriched uranium used for nuclear power plants that Iran can generate and store, as well as on its testing of more advanced centrifuge designs.

The agreement also requires Iran to remove and dismantle the core of its experimental heavy-water reactor at the Arak facility, which was designed to produce enough weapons-grade plutonium to make two bombs. (It has already poured concrete into the core of the partly finished reactor.) In its place, China is helping Iran build a new core that would produce negligible amounts of plutonium.

The agreement also put in place a rigorous monitoring system to assure compliance. The U.S. pullout wouldn’t necessarily immediately alter that inspection protocol. But if Iran ousts the inspectors or blocks their access, such a step would be considered tantamount to restoring its uranium enrichment program.

Iran could also pursue a covert weapons program that, by definition, would be outside the scope of the monitoring. But such an approach would force it to recreate the country’s entire nuclear supply chain and keep all its elements under wraps.

Will any major scientific collaborations be affected?

In a word, no. The agreement raises the possibility of creating some type of civilian research center at Fordow, but there’s been little progress toward turning that idea into reality. The agreement calls for Iran to take the first step by issuing a call for proposals on what type of laboratory would be created. Then, it is supposed to convene a workshop to explore potential research topics. However, both Iran and the Western partners have had serious concerns about the negative political fallout of such a collaboration.

Separately, the centrifuges in another wing of the Fordow facility are being converted to produce stable isotopes for commercial use in the medical and other sectors. But other projects with European partners , ranging from a center on nuclear safety to a beam line that would be installed in an Iranian light source under construction, have not moved beyond the stage of preliminary conversations. And the United States has reportedly thwarted Iran’s desire to join ITER, the international experimental fusion research being built in France.

More broadly, Trump’s decision could also affect the long-term status of Iran within the global nuclear community. “It’s important not to see JCPOA [the Joint Comprehensive Plan of Action] as the only way to link Iran to the international scientific community,” says Corey Hinderstein of the Washington, D.C.–based Nuclear Threat Initiative. As head of the Iran Task Force at the U.S. National Nuclear Security Administration until last fall, Hinderstein was part of several international meetings that focused on nuclear safety and the protection of nuclear materials. “We risk losing that engagement if the United States is no longer part of JCPOA,” she warns.