Open this photo in gallery U.S. President Donald Trump holds up a memorandum that reinstates sanctions on Iran after he announced his decision to withdraw the United States from the 2015 Iran nuclear deal on Tuesday. Chip Somodevilla/Getty Images

U.S. President Donald Trump is pulling out of the Iran nuclear deal, giving the theocratic regime in Tehran an opening to kick-start its nuclear weapons program and further distancing America from its key Western allies.

In a televised White House announcement on Tuesday afternoon, Mr. Trump said the agreement was “defective at its core” and that he would reimpose the “highest level” of sanctions.

“The fact is this was a horrible, one-sided deal that should have never, ever been made,” he said. “It didn’t bring calm, it didn’t bring peace, and it never will.”

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The move comes even as Mr. Trump seeks a similar pact with North Korea. Secretary of State Mike Pompeo flew to Pyongyang on Tuesday to lay the groundwork for a planned meeting between Mr. Trump and North Korean leader Kim Jong-un. The U.S. President is aiming to negotiate an agreement for the renegade country to give up its nuclear arsenal.

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In the wake of the announcement, Iran threatened to restart its nuclear program unless it won further guarantees from the European countries that are also part of the deal; the leaders of those countries reiterated their commitment to the pact and said it was working; and Canada lamented Mr. Trump’s decision to pull out, arguing the deal has made the world safer.

The President’s move takes place amid growing tension in the Middle East, as the U.S., Israel and Saudi Arabia seek to roll back gains made by Iran and its allies.

President Donald Trump announces the U.S. will pull out of the landmark nuclear accord with Iran, dealing a profound blow to allies and potentially deepening the President's isolation on the world stage. The Globe and Mail

The 2015 Joint Comprehensive Plan of Action, one of then-president Barack Obama’s key foreign-policy achievements, saw the United States and Europe lift crippling sanctions on Iran in exchange for Tehran dramatically restricting its uranium enrichment program and the number of centrifuges, as well as allowing International Atomic Energy Agency inspectors into the country to verify it had shut down nuclear facilities.

Mr. Trump, however, argues the JCPOA did not go far enough because it did not stop the country from interfering in Syria’s civil war or building missiles that could one day carry nuclear warheads. He said on Tuesday that he would be willing to cut a new, more comprehensive deal. “Iran’s leaders will naturally say that they refuse to negotiate a new deal. They refuse. And that’s fine. I’d probably say the same thing if I was in their position,” he said.

National security adviser John Bolton told reporters the sanctions will be phased back in over the next 180 days, giving companies time to wind up their business in Iran and leave.

Inspectors have certified that Iran has held up its end of the bargain, curtailing the possibility of the country developing a nuclear arsenal.

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Iranian President Hassan Rouhani warned Tuesday that he had put the country’s scientists on standby and they could “start enriching uranium more than before” within weeks. He signalled that he would demand more from the other Western countries in the agreement in exchange for staying in the pact.

“If we achieve the deal’s goals in co-operation with other members of the deal, it will remain in place,” Mr. Rouhani said on state television. “By exiting the deal, America has officially undermined its commitment to an international treaty.”

In a joint statement, the European signatories to the Iran deal expressed “regret and concern” about Mr. Trump’s decision. German Chancellor Angela Merkel, British Prime Minister Theresa May and French President Emmanuel Macron said they were committed to keeping the deal alive.

“Together, we emphasize our continuing commitment to the JCPOA. This agreement remains important for our shared security,” they said. “The world is a safer place as a result.”

Foreign Minister Chrystia Freeland said Canada believes the deal is essential in preventing Iran from developing nuclear weapons and to ensure global security.

“Canada regrets that the United States has decided to withdraw from the JCPOA, particularly given that, according to the IAEA, Iran continues to implement its JCPOA commitments,” she said in a statement. “Iran must not be allowed to develop nuclear weapons.”

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Ms. Freeland acknowledged that the deal is “not perfect,” but said it has helped curb what she called a threat to international peace.

A federal official, speaking on condition of anonymity, said there had been no significant Canadian sales or investments in Iran since the lifting of economic sanctions in early 2016. A prospective deal between Bombardier and an Iranian airline, reported by Iranian media, had not materialized, the official said.

Prime Minister Justin Trudeau has promised to restore diplomatic relations with Tehran – suspended by the former Harper government in 2012 over the Syrian civil war and Iran’s nuclear program – but these efforts have been hampered by the death of an Iranian-Canadian academic in an Iranian prison.

Tehran’s next move could depend on whether it feels it is still benefiting from the lifting of sanctions, said Ellen Wald, a Middle East business and political analyst. She pointed to an investment by French energy firm Total to develop a gas field in Iran as a key test: If such business deals continue, Iran will have an incentive to stay in the pact; if they fail, it could leave.

“If Iran feels it wants to renew its nuclear program, it will use [Mr. Trump’s announcement] as an excuse. I’m not convinced Iran wants to,” said Ms. Wald, author of the book Saudi, Inc.

Quitting nuclear deal: How Iran could respond Any U.S. withdrawal from the Iran nuclear agreement could prompt Tehran to retaliate by undermining the interests of Washington and its allies in the Middle East. IRAN Iranian President Hassan Rouhani has warned U.S. to stay in nuclear deal or face “severe consequences” Iran is threatening to withdraw completely from Non-Proliferation treaty, designed to stop spread of nuclear weapons SYRIA Israel says Iran has recruited at least 80,000 Shia paramilitary fighters to shore up government of President Bashar al-Assad If nuclear deal falls through, Iran will have little incentive to stop Shia militia allies in Syria from carrying out attacks against Israel LEBANON Iran-backed Hezbollah and its allies won over half of seats in May parliamentary election Iran could pressure Hezbollah leader Hassan Nasrallah to isolate its political opponents and destabilize Lebanon IRAQ Al-Fatah coalition, led by pro-Iranian former militia leader Hadi al-Amiri, comprises militia groups of Popular Mobilization Forces (PMF). Influential faction vying for power in upcoming parliamentary elections Iran could encourage PMF factions to step up attacks against some 5,200 U.S. troops that remain in Iraq YEMEN U.S. and SaudI officials say Iran is supplying rebel Houthi fighters with missiles Iran could increase support for Houthis, possibly provoking military response from Saudi Arabia and United Arab Emirates THE GLOBE AND MAIL, SOURCE: GRAPHIC NEWS; REUTERS PHOTOS: NEWSCOM; GETTY Quitting nuclear deal: How Iran could respond Any U.S. withdrawal from the Iran nuclear agreement could prompt Tehran to retaliate by undermining the interests of Washington and its allies in the Middle East. IRAN Iranian President Hassan Rouhani has warned U.S. to stay in nuclear deal or face “severe consequences” Iran is threatening to withdraw completely from Non-Proliferation treaty, designed to stop spread of nuclear weapons SYRIA Israel says Iran has recruited at least 80,000 Shia paramilitary fighters to shore up government of President Bashar al-Assad If nuclear deal falls through, Iran will have little incentive to stop Shia militia allies in Syria from carrying out attacks against Israel LEBANON Iran-backed Hezbollah and its allies won over half of seats in May parliamentary election Iran could pressure Hezbollah leader Hassan Nasrallah to isolate its political opponents and destabilize Lebanon IRAQ Al-Fatah coalition, led by pro-Iranian former militia leader Hadi al-Amiri, comprises militia groups of Popular Mobilization Forces (PMF). Influential faction vying for power in upcoming parliamentary elections Iran could encourage PMF factions to step up attacks against some 5,200 U.S. troops that remain in Iraq YEMEN U.S. and SaudI officials say Iran is supplying rebel Houthi fighters with missiles Iran could increase support for Houthis, possibly provoking military response from Saudi Arabia and United Arab Emirates THE GLOBE AND MAIL, SOURCE: GRAPHIC NEWS; REUTERS PHOTOS: NEWSCOM; GETTY Quitting nuclear deal: How Iran could respond Any U.S. withdrawal from the Iran nuclear agreement could prompt Tehran to retaliate by undermining the interests of Washington and its allies in the Middle East. IRAN Iranian President Hassan Rouhani has warned U.S. to stay in nuclear deal or face “severe consequences” Iran is threatening to withdraw completely from Non-Proliferation treaty, designed to stop spread of nuclear weapons SYRIA Israel says Iran has recruited at least 80,000 Shia paramilitary fighters to shore up government of President Bashar al-Assad If nuclear deal falls through, Iran will have little incentive to stop Shia militia allies in Syria from carrying out attacks against Israel LEBANON Iran-backed Hezbollah and its allies won over half of seats in May parliamentary election Iran could pressure Hezbollah leader Hassan Nasrallah to isolate its political opponents and destabilize Lebanon IRAQ Al-Fatah coalition, led by pro-Iranian former militia leader Hadi al-Amiri, comprises militia groups of Popular Mobilization Forces (PMF). Influential faction vying for power in upcoming parliamentary elections Iran could encourage PMF factions to step up attacks against some 5,200 U.S. troops that remain in Iraq YEMEN U.S. and SaudI officials say Iran is supplying rebel Houthi fighters with missiles Iran could increase support for Houthis, possibly provoking military response from Saudi Arabia and United Arab Emirates THE GLOBE AND MAIL, SOURCE: GRAPHIC NEWS; REUTERS PHOTOS: NEWSCOM; GETTY

Mr. Trump repeatedly promised to quit the deal during his presidential campaign, portraying it as a generous giveaway to Iran by Mr. Obama.

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Still, he hesitated for more than a year after taking office. Mr. Trump acted as Iran’s influence across the region appeared to grow: Tehran-aligned Syrian dictator Bashar al-Assad has been pushing back opposition forces in his country’s long and bloody civil war, while Hezbollah, another Iranian ally, surged in Lebanese elections earlier this week.

“This wasn’t about Iran’s nuclear program; they were in compliance with the deal,” said Doga Eralp, an expert in international conflict resolution at American University in Washington. “It’s really about punishing Iran for turning itself into a kingmaker in Middle East politics.”

The policy dovetails with Mr. Trump’s “America first” move away from multilateral deal-making and the international order it has helped build since the Second World War.

In a statement on Facebook, Mr. Obama slammed the move. “The consistent flouting of agreements that our country is a party to risks eroding America’s credibility, and puts us at odds with the world’s major powers,” he wrote.

And it risks making Washington seem an unreliable negotiating partner, even as it heads into talks with Pyongyang.

“The U.S., as the world’s superpower, is not able to keep its promises,” Prof. Eralp said of the message to Mr. Kim. “It is willing to change its positions.”