U.S. President Donald Trump, making good on a pledge from the 2016 election campaign, announced on Tuesday he will reimpose sanctions on Iran, pulling the U.S. out of a landmark international nuclear accord with Iran he called a one-sided "catastrophe."

Although arms-control experts widely acknowledge the deal signed by major world powers in 2015 is imperfect, they warned that its collapse would give the Islamic republic the green light to restart nuclear activities, potentially triggering a nuclear arms race in the Middle East.

Here are some important effects of Trump's announcement.

Is the Iran deal now dead?

Not yet. There are other world powers who signed the Joint Comprehensive Plan of Action, or JCPOA, in 2015 that are still involved, including the remaining four permanent members of the UN Security Council — Russia, China, Britain, France — and the whole of the Europe Union.

As Suzanne Maloney, deputy director of the foreign policy program at the Brookings Institution, noted in a phone briefing with reporters, the deal isn't a treaty. It's an international political agreement. That means the U.S. voluntarily bought into it, and it doesn't necessarily mean the whole thing falls apart.

France, Germany, and the UK regret the U.S. decision to leave the JCPOA. The nuclear non-proliferation regime is at stake. —@EmmanuelMacron

"One of the peculiarities of the agreement," Maloney told reporters, is it "does not include mechanisms for withdrawal."

That said, the most effective sanctions are the American sanctions against Iran, so the threat of it falling apart is very real, and the onus may be on European partners to try to salvage the arrangement, said Behnam Ben Taleblu, an expert on Iranian security with the Foundation for Defense of Democracies.

"How do you save a deal that just lost the most powerful economy in the world?" he asked.

When the U.S. and other nations signed the deal in 2015, they agreed to end a number of punitive sanctions against Iran and allow the resumption of oil and other exports to the West. In return, Iran agreed to cut its enriched uranium stockpile by 98 per cent and to remove two-thirds of its centrifuges (devices that can enrich uranium to the point it can be used to make nuclear weapons).

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Some of the sanctions that were waived as part of the deal were due for reassessment in July, but it appears Trump's reference to imposing the "highest level" of economic sanctions means what arms-control wonks refer to as a "hard withdrawal," or "hard exit."

Trump displays a presidential memorandum after announcing his intent to withdraw from the JCPOA. (Jonathan Ernst/Reuters)

"They are bringing back literally everything the [JCPOA] lifted. The big message of the waiver this time is, you should wind down your business with Iran," said Taleblu.

"This wasn't joking. This was very serious, very fast. Akin to grabbing the Iranians by the shirt collar and saying, 'Hey, stop it.'"

How does this affect Mideast stability?

The main worry is that Iran will restart nuclear activities, feeling no obligation to respect the deal's terms.

A nuclear ramp-up by Shia-dominated Iran could plunge the Middle East into a state of regional insecurity, possibly sparking a nuclear arms race as skittish Sunni powers consider whether they should also pursue their own nuclear weapons.

Saudi Arabia, whose leader, Crown Prince Mohammed bin Salman, is shown above, has said it will match Iran's nuclear program, and some observers fear it will make good on that promise now that the U.S. has pulled out of the nuclear deal with Iran. (Bandar Algaloud/Saudi Royal Court/Reuters)

"Saudi Arabia has already said that it will match Iran's program with a bomb program of its own," said Joseph Cirincione, president of Ploughshares Fund, a global security foundation.

"So, the nuclear arms race that we had stopped with this agreement in 2015 will be back on."

The president said today he's withdrawing from the 2015 nuclear deal 11:11

Egypt could obtain a nuclear weapon, he said, though it is economically constrained. And Cirincione said the Saudis may also help the United Arab Emirates acquire their own bombs.

The Israeli newspaper Haaretz reported shortly after Trump's announcement that the Israeli military is bracing for an imminent attack by Iran.

What role is Israel playing?

Israeli Prime Minster Benjamin Netanyahu's PowerPoint presentation, broadcast on live TV last month and pointing to intelligence smuggled by Mossad out of Iran, offered what Trump said on Tuesday was "definitive proof that this Iranian promise was a lie."

Netanyahu showed documents he said provided proof of Iranian duplicity, saying Iran had secreted a cache of documents he called "atomic archives" to a covert location in Tehran's Shorabad district.

The atomic archives, according to Netanyahu, were being hidden for future use so Iran could later resume its pursuit of nuclear bombs.

The Israeli leader's presentation advancing his skepticism about Iran acting in good faith in accordance with the JCPOA "may not have been what broke the camel's back" for Trump, Taleblu said. "But it was another straw, if you will."

U.S. President Donald Trump has pulled America from the Iran nuclear deal, in a move to reinstate strict economic sanctions. 9:22

How about U.S. global influence?

To Brett Bruen, a former director of global engagement during the Obama administration, withdrawing from the deal alienates the U.S. from its allies, who aim to keep the JCPOA afloat.

"You end up in a situation where America has been shown to carry less influence than at any time after World War II, because the world is uniting and strengthening its positions and diminishing that of the U.S.," he said. "It really matters less where America is right now."

David Barrows protests outside the White House as Trump announces the U.S. withdrawal from the Iran deal. (Leah Millis/Reuters)

How does this affect North Korea?

The timing is awkward, coming just as North Korea needs to know whether it can trust the U.S. to keep its word ahead of a summit on denuclearization. But it's also bigger than just North Korea, says Bruen.

Withdrawing from the landmark accord throws into doubt any confidence nations could have in U.S. foreign policy.

"When we're asking them to make compromises, whether it's dismantling a nuclear program or building a trade agreement … it requires those countries to believe that when the U.S. signs its name on the dotted line, it means something."

How does this affect Iran domestically?

Iranian hardliners who never trusted the Americans in the first place will feel vindicated while moderate Iranian president Hassan Rouhani's hand will be weakened.

"The hardliners always said America is the devil, and you can't trust them. This proves them right," Cirincione said. "The bigger picture is, it wrecks any possibility, however slim, of any kind of reconciliation with America."

In a statement on Tuesday following Trump's announcement, Rouhani said, "We have been saying since 40 years ago …that Iran is a country that adheres to its commitments, and the U.S. is a country that has never adhered to its commitments."

Taleblu said., "Anything that affects the JCPOA in Iran will affect Rouhani's standing. He's intertwined with that deal."

What's the alternative?

John Bolton, Trump's new national security adviser, has made it clear that regime change in Iran is his solution. Speaking in Paris in 2017 to the fringe dissident group Mujahedeen Khalq, or the M.E.K., Bolton said the "only solution is to change the regime itself."

"And that's why before 2019, we here will celebrate in Tehran," he said.

Until 2012, the U.S. considered the M.E.K. a terrorist organization.

Cirincione sees Bolton's proposal as wildly naive. Even with its flaws, he said, having a deal is better than not having a deal for the time being.

"Even if you believed that Iran's intention was to build a bomb in 15 years, why end the agreement now? Why not wait for 15 years and then take whatever action you want to take?

"Now we're in a situation where the U.S. has fewer tools, fewer allies, Iran is the victim, and America is the bully."

Watch Trump's full announcement of U.S. withdrawal from Iran deal: