President Donald Trump just made one of the most consequential decisions of his presidency.

Around 2 pm on Tuesday, Trump withdrew the US from the Iran nuclear deal, saying the US would reimpose sanctions on Tehran and stop other countries from improving ties with the country.

Trump signaled his displeasure with the Iran nuclear deal for months, at one point calling it “the worst deal ever.” European allies like France and Germany have tried to convince Trump to stay in the deal by promising to improve the agreement, but it’s anyone’s guess whether that’s enough to convince Trump to stay in.

The Iran deal, and why Trump decided to pull the US out of it, is incredibly complicated, so it’s understandable if you’re a little bit lost right now.

Don’t sweat it — we’ve got you covered. Here are answers to some of the most basic questions about the Iran nuclear deal that will help you get up to speed before Trump speaks this afternoon.

1) What does the Iran deal do?

Simply put, the nuclear deal, known formally as the Joint Comprehensive Plan of Action, stops Iran from getting a nuclear weapon for at least a decade.

On July 14, 2015, a group of countries — the United States, the United Kingdom, France, Russia, China, Germany, and the European Union — agreed to lift crippling sanctions imposed on Iran’s nuclear program, giving it greater access to the global economy.

In return, Iran agreed to take concrete steps to curb its nuclear program, limiting it to strictly peaceful applications, and to allow comprehensive inspections of key nuclear facilities by the International Atomic Energy Agency (IAEA) to ensure compliance.

Here are just some of those steps:

Iran had about 20,000 centrifuges, but the deal said Iran can’t use more than about 5,000 of them — and they must be among the oldest and least useful ones. That makes it very hard for Iran to create a nuclear bomb if it wanted to today.

Iran gave up 97 percent of its enriched uranium but kept only 300 kilograms of its 10,000-kilogram stockpile in its current form. It has to keep its uranium at about 3.67 percent enrichment; uranium used in nuclear bombs must reach 90 percent enrichment.

Iran would destroy or export the core of its plutonium plant at Arak and replace it with a new core that cannot produce weapons-grade plutonium.

What all that means: Before the deal, Iran could likely make a nuclear bomb within two or three months if it decided to. But after the accord, it would take Iran about a year to make that weapon.

So that’s it: Iran rolls back its ability to get a nuclear weapon and more money flows into the country. There was nothing in the deal that said Iran couldn’t do other nefarious things, like test missiles, sponsor terrorist groups such as Hezbollah and Hamas, or crack down on human rights at home.

Still, there’s controversy: The restrictions on Iran’s centrifuges disappear after 10 years, and the limits on uranium enrichment go away five years after that. Some critics — including Trump — believe it’s therefore possible Iran could go back on the nuclear path around the mid-2020s.

However, inspectors will continue to have access to Iran’s nuclear facilities even after those terms expire, meaning they might still catch Tehran if it tries to make a nuke then.

2) Why would Iran want a nuclear weapon?

Officially, Iran says it has never tried to make a nuclear weapon. “We never wanted to produce a bomb,” Iranian Foreign Minister Javad Zarif said on April 22 on CBS News’s Face the Nation.

We know that’s not true. Just over a week later, Israeli Prime Minister Benjamin Netanyahu presented evidence from thousands of Iranian documents obtained in an intelligence coup that Iran was on the path to a nuclear weapon.

And there are two main reasons Iran might want a nuclear weapon: for defensive purposes and for offensive purposes.

Let’s start with defense. Leaders in the United States and Israel continually threaten to bomb Iran. The Iranian military is strong, but it would likely lose in a war with Washington or Jerusalem.

But having nukes is an equalizer and would help Iran deter any American and Israeli plans to attack. (This is similar to North Korea’s reason for having nuclear weapons, by the way.)

Having nuclear bombs would also give Tehran flexibility to wreak even more havoc in the Middle East. With those weapons, Iran could more boldly support Bashar al-Assad in Syria and the terrorist groups like Hezbollah and Hamas that it finances, or confront its archenemy Saudi Arabia with more force. In effect, Iran could be more aggressive and pursue some of these objectives with less worry that other countries will attack it.

Basically, having nuclear weapons would make Iran stronger, as it would any country. That, in part, is why successive US presidents have worried about Iran getting the bomb.

Former President Barack Obama decided that making a deal with Iran was the best way to prevent that from happening.

3) Why did the US sign the Iran deal?

When it came to Iran and nuclear weapons, Obama had two pretty clear stances.

Here’s his first: “My goal, when I came into office, was to make sure that Iran did not get a nuclear weapon and thereby trigger a nuclear arms race in the most volatile part of the world,” Obama told NPR’s Steve Inskeep on April 7, 2015, three months before the US and six world powers signed the Iran deal.

And here’s the second: “There really are only two alternatives here: Either Iran getting a nuclear weapon is resolved diplomatically through negotiation,” Obama said at the White House on July 15, 2015 — the day after the agreement’s completion, “or it is resolved through force, through war. Those are the options.”

To put that all together, Obama didn’t want Tehran to acquire nukes, and he claimed the only way to stop Iran from getting them was either to strike a deal or strike it with bombs.

A war with Iran, of course, would be very nasty. As my colleague Zack Beauchamp reported, the US would need to “destroy Iran’s air defenses, including fighters and surface-to-air missiles, in order to ensure the bombs hit their targets and to prevent Iran from doing serious damage in response.” And the US would repeatedly need to hit multiple Iranian targets, including nuclear and centrifuge production facilities.

Here’s the rub, though: Experts doubt that airstrikes alone could end Iran’s nuclear program. And it gets worse. Experts at the Wilson Center think tank, after reviewing military studies on the issue, concluded that even if the US military carried out strikes “to near perfection,” the best-case scenario would still be only a four-year delay in Iran’s progress toward a nuclear weapon.

That all sounds bad, and perhaps explains why Obama said diplomacy proved a better option than war to keep Iran from getting the bomb.

Trump, however, thinks Obama’s gambit failed.

4) Why does Trump want to leave the Iran deal?

Trump has railed against the Iran deal since the early days of the presidential campaign. He called the agreement “the worst deal ever” and “one of the worst and most one-sided transactions the United States has ever entered into.”

There are three main reasons why.

First, Trump says Iran is violating the “spirit” of the deal, if not the letter, mostly because Iran continues to work against America in the Middle East. In essence, Iran remains a US enemy, and Obama failed to address that with the deal.

But Trump’s objection misses the point. “You could say it’s a terrible deal because it doesn’t cover Hezbollah, and Syria, and Yemen, and missiles, and human rights,” Ernest Moniz, Obama’s energy secretary and top Iran deal negotiator, told me last August. “That’s not what the agreement is.”

Take the missiles part: The Iran deal doesn’t prohibit Iran from testing missiles. At all.

There is a totally separate UN Security Council resolution that includes language specifically about Iran’s ballistic missiles, and Iran is probably violating at least the spirit of that resolution, if not the letter of it.

But again, that’s separate from the Iran nuclear deal itself, which only targets Tehran’s nuclear program. That was by design: The Obama administration knew that to get Iran to agree to curb its path to a nuclear bomb, the US would have to give up a lot more than simply lifting a few sanctions.

Second, Trump doesn’t like that certain restrictions of the deal — like the ones on uranium enrichment and the use of centrifuges — end. He would prefer those restrictions remain indefinitely so it’s even harder for Iran to obtain a nuclear weapon.

All of that leads to Trump’s third reason: He thinks he could make a better deal. In fact, he’s signaled a desire to improve the Iran deal, not necessarily leave it — yet. “I have outlined two possible paths forward: either fix the deal’s disastrous flaws, or the United States will withdraw,” Trump said in January. Here’s how he wants to do it (this will sound familiar): curb Iran’s missile tests, allow more inspections of Iranian nuclear facilities, and remove the time limits on uranium and centrifuge restrictions.

The Trump administration is in talks with European officials on Trump’s proposed changes, but they have yet to reach an agreement. Meanwhile, a parade of European leaders — French President Emmanuel Macron, German Chancellor Angela Merkel, and UK Foreign Minister Boris Johnson — have personally appealed to Trump to remain in the agreement.

Macron, for one, is pessimistic he succeeded. After visiting Trump at the White House in April, Macron told US reporters he thought Trump would leave the deal.

5) Has Iran broken the nuclear deal?

No. (But a little bit yes.)

According to the IAEA, the UN’s nuclear inspectors, Iran continues to comply with every part of the nuclear deal. “As of today, I can state that Iran is implementing its nuclear-related commitments,” IAEA Director General Yukiya Amano said on March 5.

In effect, Iran is no closer to having a nuclear weapon now than when it signed the accord back in 2015.

But it has slightly violated some of the technical terms of the deal over the past few years. For example, in 2016 Iran twice exceeded that amount of heavy water in nuclear reactors it could have. But the country came clean about this to IAEA inspectors and then promptly shut down that reactor.

But that’s about it. As of now, Iran still can’t acquire a nuclear weapon anytime soon. In other words, the Iran deal — for now — is working.

6) What happens if the US leaves the Iran deal?

The very first thing that would happen is that the US would reimpose the sanctions it lifted on Iran’s nuclear program and possibly take other retaliatory actions.

But what happens after that is not totally clear. How will the other countries that are party to the deal react? How will Iran react?

We just don’t know.

European signatories have said they have no intention of leaving the deal even if America does. That’s in their interest since European businesses have started to, well, do business with Iran since the deal was signed. If the deal ends, it’s possible that Europe would have to end its newfound economic ties to Tehran.

As for Iran, Iranian President Hassan Rouhani said on Monday that the deal could survive even without America. “If we can get what we want from a deal without America, then Iran will continue to remain committed to the deal,” Rouhani said. “What Iran wants is our interests to be guaranteed by non-American signatories,” adding that “[g]etting rid of America’s mischievous presence will be fine for Iran.”

That contradicts what Iranian Foreign Minister Javad Zarif said last week, when he noted that Tehran would “most likely” leave the deal if the US did.

And if it does, Iran might put itself on the path to a nuclear weapon again.

So those are the stakes: angry, potentially worse-off European allies and an Iran possibly unleashed to build nuclear weapons.

French President Macron said on Sunday that the US leaving the Iran deal “could mean war,” although he added that he doesn’t think Trump wants a conflict.

Still, that shows just how serious this moment is: Foreign leaders are openly wondering if war with Iran is imminent because of Trump’s decision.