WASHINGTON—During his first year in office, President Donald Trump often acceded to the advice of cautious national security aides. At their urging, he added troops to the war in Afghanistan, delayed plans to move the U.S. Embassy in Israel to Jerusalem and preserved a nuclear-containment deal with Iran even though he reviled it.
Now, Mr. Trump has come to trust that his own efforts to prod, cajole and intimidate global rivals can do what his predecessors couldn’t, according to people familiar with his thinking. With a new national security team at his side, Mr. Trump is moving ahead with efforts to dismantle pillars of former President Barack Obama’s agenda, pushing the boundaries of traditional foreign policy and national-security decision making in the process.
On Tuesday, he took his latest such step by dismissing appeals from European allies and pulling out of Mr. Obama’s 2015 agreement that curbed Iran’s nuclear program.
“It’s the Art of the Deal: Refuse to be bound by existing agreements that are not in America’s interest. Exit, or threaten to exit, these agreements and use maximum pressure in order to get a better deal,” said Mark Dubowitz, chief executive of the Foundation for Defense of Democracies, who pushed for major changes to the Iran deal, not scrapping it. “It’s high risk. It can lead to big successes or big failures.”
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The potential risks of the approach became evident in the hours after Mr. Trump announced the U.S. shift back toward a more confrontational approach to Iran, when Syrian officials said Israel carried out another series of airstrikes on Iranian targets near Damascus. Israel warned residents in the Golan Heights that Iran might strike, raising fears of more conflict in the Middle East.
Mr. Trump’s approach has fueled criticism from those who question his strategy. Leon Panetta, who served as Mr. Obama’s Central Intelligence Agency director and his defense secretary, said Mr. Trump appears intent on dismantling his predecessor’s achievements without having a vision for what should take its place.
“The result is that he takes steps which create a great deal of chaos without any kind of strategy as to what you do next,” said Mr. Panetta. “If you don’t have a strategy as to where you go after you roll a grenade in the room it makes it pretty dangerous.”
Mr. Trump’s allies acknowledge the dangers posed by the president’s approach. But they say the tactics are already paying dividends, especially by pushing Kim Jong Un, North Korea’s mercurial leader, into talks with the U.S.
For months, Mr. Trump stoked fears of war by belittling Mr. Kim as “Rocket Man” and threatening him with “fire and fury.” Mr. Trump pressed his national security team to develop options for striking North Korea, an approach publicly embraced by his new national security adviser, John Bolton.
Critics of Mr. Trump’s Iran decision Tuesday argue that it makes it less likely that North Korea will agree to a deal with the U.S. But Mr. Bolton said that the move would strengthen the U.S. hand in talks.
“It sends a clear signal that the United States will not accept inadequate deals,” he said.
The confrontational approach with North Korea raised fears of a nuclear war, but Mr. Trump’s supporters say the threats rattled Mr. Kim.
“I do think Kim Jong Un is worried about Trump,” said Mr. Dubowitz. “He [Mr. Kim] may think he can play him, but he also needs to be concerned that Trump could go where no U.S. president has gone with respect to maximum pressure. Trump could be a Twitter tiger or he could be a ferocious one. Kim Jong Un will learn more soon.”
Mr. Trump’s allies said the president viewed his approach to North Korea as a potential template for Iran: Use threats to intimidate an adversary, press wary allies into taking tough steps to isolate the troubling rival and force people to the negotiating table.
Mr. Trump has surrounded himself with a revamped national security team intent on harnessing the president’s style and using it to implement vows he made on the campaign trail, according to people familiar with the president’s thinking.
But the administration faces an uphill battle in persuading European allies to work with the U.S. after dismissing its attempts to save the deal with Iran. European leaders, led by French President Emmanuel Macron, quickly distanced themselves from the U.S. decision as they vowed to work with Iran to salvage the agreement.
Mr. Trump is relying more on Israel and Saudi Arabia, Iran’s two biggest rivals in the Middle East, to spearhead new efforts to blunt Tehran’s regional influence. Both are in open conflict with Iran or its proxies, and some analysts said Tuesday’s announcement increased the risks of broader regional clashes.
Now surrounded by more like-minded national security advisers, Mr. Trump is increasingly challenging the status quo. He wants U.S. troops out of Syria, and has voiced similar hopes for Afghanistan. Next week, U.S. officials will officially open the U.S. Embassy in Jerusalem—a move Trump administration officials once said might not happen for years.
And now, Mr. Trump has scrapped the deal with Iran that even members of his national security team have told him was working.
“We’re not an imperial power,” said one former Trump administration official. “He doesn’t want American kids dying in the Hindu Kush while taxpayers underwrite it. America First means the opposite of isolationism—where it’s in our vital national security Trump wants allies—not protectorates.”
To some, Mr. Trump’s approach to foreign policy is reminiscent of President Ronald Reagan’s strategy for confronting the Soviet Union in the 1980s. Mr. Reagan belittled Moscow as the “Evil Empire” and was once caught on microphone joking about bombing the Soviet Union in five minutes.
Mr. Reagan’s style stoked fears of nuclear war, but he eventually negotiated a historic and unexpected agreement to reduce the number of nuclear weapons built by the U.S. and Soviet Union.
There were differences. While Mr. Reagan relied on the recommendations of his seasoned national security team, Mr. Trump has repeatedly chafed at the guidance of his own advisers, leading to a constant churn of top officials.
“Ronald Reagan was smart enough to have smart people working in the key jobs, people who know what the hell they were doing—and he was willing to listen to them,” said Mr. Panetta. “I think that’s been a problem for this president—trying to not only get the smartest people to be part of his team, but then to stick with them.”
Write to Dion Nissenbaum at dion.nissenbaum@wsj.com