Donald Trump has never been a big fan of the Iran nuclear deal. Today (May 8) he announced that the US will pull out of the 2015 international accord, and that he plans to impose “the highest level of economic sanctions” on Iran. The move could have big consequences for the world’s nuclear landscape and for global diplomacy.

Under the Iran accord, also known as the Joint Comprehensive Plan of Action (JCPOA), six countries—the US, the UK, China, Russia, Germany, and France—had agreed to lift economic sanctions on Iran in order to keep a check on Iran’s nuclear capabilities. The International Atomic Energy Agency (IAEA), the organization assigned to watch over Iran’s nuclear activities, says the country has respected the terms of the deal, producing only a limited amount of nuclear fuel for its power plant.

Apart from the US, the countries that signed the agreement would like to keep the deal in place. But Trump and his advisers say it doesn’t go far enough. Since the deal was signed, Iran has launched ballistic missiles and been accused of continuing to fund terrorism in the Middle East. In Trump’s opinion, any deal with Iran should have provisions to bring these activities under control too.

Iran has indicated that it would like to keep the deal in place, so long as other member countries agree to stick with it. But if the other countries decide to pull out, Iran will be free to restart its nuclear-weapons program.

Iran’s foreign minister, Ali Akbar Salehi, warned in April that it could restart its nuclear-enrichment program in less than four days. Quartz spoke to experts to assess the credibility of those claims, as well as other potential risks posed by Iran.

What does it take to build a nuclear bomb?

Building a nuclear bomb isn’t easy. That’s because the main ingredient in most atomic bombs is a highly unstable form of uranium called U-235.

In its natural form, uranium is mostly a mixture of two isotopes: U-238 (99.2%) and U-235 (0.7%). When the Iran deal was signed in 2015, Iran was capable of enriching uranium—that is, increasing the concentration of U-235 in a mixture—to 20%.

But building a nuclear weapon requires at least 80% enriched uranium, and preferably 95%. That’s a complicated task.

Here’s how the process of enriching uranium works: Uranium ore is extracted from a mine. It is then milled and chemically processed to concentrate uranium oxide—a compound of uranium and oxygen.

This solid is chemically transformed to uranium hexafluoride—a compound of uranium and fluorine—which can be easily heated to its gaseous state. Next, the gas is injected into huge spinning vessels called centrifuges. These are capable of separating a mixture based on small differences in their weight. U-235, which has three fewer neutrons, is separated from U-238. But each centrifuge can only enrich the mixture a little in each cycle. Thousands of centrifuges need to be built and operated to collect quantities that go in a bomb.

In 2015, the Iran deal forced Iran to cut the number of centrifuges it had built from 19,000 to 5,000. And Iran’s centrifuges aren’t very good. “They are slow, outdated by decades—and cheap knockoffs,” says Max Fisher, a writer for the New York Times.

The deal also meant Iran had to stop researching advanced centrifuges and give up its stock of 20% enriched uranium. The centrifuges that are still running are heavily monitored, and never produce enriched uranium beyond 3.67%, according to the IAEA, which is used for running nuclear power plants and other research activities.

Does Iran pose a serious nuclear threat?

If the deal falls apart, Iran could theoretically restart its nuclear program pretty quickly—although no expert could say whether it would be possible to do so within four days. But it will have a lot of lost ground to make up for, says Scott Lucas, professor of international politics at the University of Birmingham. And even if Iran managed to get back to the nuclear capabilities it had before the deal, however, it’s not anywhere close to building a nuclear bomb.

“With the safeguards in place today, it will take Iran at least a year to get enough enriched uranium,” says Bryan Gibson, professor of history at Hawaii Pacific University. “That puts them more than 10 years behind North Korea, who detonated their first atomic bomb in 2006.”

Building the bomb would also require Iran to master handling unstable uranium. The atomic bomb punches so much power because of a chain reaction. It starts with a single neutron hitting an atom of U-235 and causing it to split, releasing more than one neutron in the process, which then goes to split two more atoms of U-235, and so on.

To start the process, however, you need a non-nuclear explosion that can kick a few neutrons with enough speed to hit a uranium nucleus with sufficient force. All the while, the enriched uranium must be held in close proximity for long enough for the chain reaction to grow. It’s not an easy feat of engineering.

Would Iran renege on the deal?

Both Lucas and Gibson think that it’s unlikely Iran would go back to enriching uranium, beyond what is allowed in the JCPOA. “There is a desire among Iranian people to engage the international community, to trade, to grow its economy, to use its educational prowess, and to make Iran a regional power,” says Gibson. “By US walking away from the deal, I don’t think that desire will go away. Worse, Iran would lose the moral high ground because it has so far followed the conditions of the deal in good faith.”

When Iran was enriching uranium to 20% before 2015, the country wasn’t actually able to do much with it. “It was a way of antagonizing the West,” says Gibson. The goal was to get bigger global powers to treat Iran as an important country.

On Monday (May 7), Hassan Rouhani, the president of Iran, said in a speech: “If our goals in JCPOA can be achieved without the United States, that’s better. Otherwise, we will make our decision.” Lucas suggests this is Rouhani’s way of courting the other five global superpowers that signed the agreement. If the US pulls out, it’s likely Iran will still try to save the deal by convincing other member countries to stay.

What happens if the deal falls apart?

If the US pulls out of the JCPOA, the European countries—the UK, France, and Germany—will be in a difficult position, forced to choose between Iran and their ally. That’s why there is a real chance that the deal could fall apart.

“If you strike a deal with Iran and get it to adhere for 10 years, you [have an example for] how the world could behave over nuclear programs,” says Lucas. That is why Emmanuel Macron, France’s president, didn’t mince his words in his reaction to Trump’s decision. “France, Germany, and the UK regret the U.S. decision to leave the JCPOA,” he tweeted. “The nuclear non-proliferation regime is at stake.”

This article was updated after Trump’s announcement at 2pm ET.