The New York Times| MARK LANDLER: When President Trump took a phone call from the leader of Taiwan in December and asserted that the United States might no longer be bound by the “One China” policy, his defenders hailed it as a show of strength — the latest delicate issue on which Mr. Trump was willing to challenge decades of diplomatic orthodoxy.


On Thursday evening, Mr. Trump fell back into line. In a call with President Xi Jinping of China, he pledged fealty to One China, a 44-year-old policy under which the United States recognized a single Chinese government in Beijing and severed its diplomatic ties with Taiwan.


Mr. Trump has also tacked to the center on Israel. After presenting himself as a stalwart defender of Prime Minister Benjamin Netanyahu who would buck the pressure campaign against Israeli settlements in the West Bank, Mr. Trump warned Israelis this week that he did not believe that “going ahead with these settlements is a good thing for peace.”


And on Iran, where Mr. Trump threatened as a candidate to rip up the nuclear deal struck by President Barack Obama, advisers to the new president told the European Union’s top foreign policy official, Federica Mogherini, that the United States would fully carry out the agreement.


As Mr. Trump begins to shape his foreign policy, he is proving to be less of a radical than either his campaign statements or his tempestuous early phone calls with foreign leaders would suggest. On Friday, as he welcomed Prime Minister Shinzo Abe of Japan to the White House, Mr. Trump characterized America’s alliance with Japan as a “cornerstone of peace and stability.” Those time-tested words bore little resemblance to his threats during the campaign to mothball the partnership.


“Every president discovers that it looks different from the perspective of the Oval Office than it did on the campaign trail,” said Martin S. Indyk, the executive vice president of the Brookings Institution. “The fact that President Trump is proving flexible on some key foreign policy issues suggests he’s less ideologically driven than his early moves would imply.”


To some extent, Mr. Trump is simply undergoing the same evolution that all of his predecessors went through. Mr. Obama, who ran as an antiwar candidate, became an avid user of drone strikes and other covert counterterrorism operations pioneered by George W. Bush.


In Mr. Trump’s case, however, the recalibration is starker because of the extreme nature of the positions he had staked out on issues like China, Russia and the NATO alliance, as well as the Trump campaign’s thin ranks of policy advisers and his slowness in assembling a full national security team in the White House. It also stands in stark contrast to his more uncompromising approach on other matters like the legal challenges to his executive order on immigration.


“He made it all the way to inauguration without doing the deep-dive policy reviews and internal debates that every other successful administration does during the campaign and the transition,” said Peter D. Feaver, who served in Mr. Bush’s National Security Council.


Secretary of State Rex W. Tillerson, administration officials said, was among those who urged Mr. Trump to publicly endorse the One China policy as a way to defuse tensions with Mr. Xi. Before Thursday, the two leaders had not spoken since Nov. 14; administration officials said that the Chinese leader would not get on the phone with Mr. Trump without assurances from the administration that he would commit to the policy.


Defense Secretary Jim Mattis has also emerged as an influential player, officials said. He recently returned from a trip to Asia during which he offered reassurances of American support for allies like Japan and South Korea. Among his travel companions was Matthew Pottinger, who recently became the senior director for Asia in the National Security Council.


“There finally is an administration beginning to take shape around him, which there was not before — Tillerson and Mattis in particular,” said Jeffrey A. Bader, a former top China adviser to Mr. Obama. “During the transition, no one had the nerve or expertise to contradict him.”


In addition to his cabinet officers, Mr. Trump and his aides are beginning to soak up advice from other leaders. The president stiffened his tone on settlements after he met briefly with King Abdullah II of Jordan; his advisers, including Jared Kushner, spoke with Arab officials, who urged the administration not to give Israel a free hand on the issue.


In an interview with an Israeli newspaper, Israel Hayom, which was published on Friday, Mr. Trump said settlements “don’t help the process” — a phrase that is not substantially different from those used by Mr. Obama or Mr. Bush.


“Every time you take land for settlements, there is less land left,” Mr. Trump continued.


When the White House issued an initial statement on settlements last week, some analysts read its ambiguous wording as leaving Israel plenty of room for maneuvering. It said that settlements were not an impediment to peace, but that they “may not be helpful.” Mr. Trump’s remarks were more pointed, however, and experts said they suggested a genuine shift.


“The right-wingers who looked at him like he was going to be the master builder of the Middle East — that was not warranted,” said David Makovsky, a senior fellow at the Washington Institute for Near East Policy. “The more he immerses himself in this, he will see there are conflicting pressures and realities. He is going to have to navigate within those constraints.”


The White House’s pledge to carry out the Iran nuclear deal does not reflect a change of heart on Tehran. The administration last week imposed new sanctions on Iran for its launching of a ballistic missile. But officials in the White House are more focused on curbing what they see as a pattern of aggressive behavior by Iran from Syria to Yemen.


Mr. Trump’s retreat on One China, experts said, also should be seen in a regional context. He offered the concession to Mr. Xi on the eve of a three-day visit by Mr. Abe, during which the Japanese leader had lunch at the White House and is Mr. Trump’s guest at Mar-a-Lago, where they and their wives are having dinner, and the two men are playing golf at a nearby club.


To allow the tensions between China and the United States to fester during such a prominent display of hospitality to the Japanese, analysts said, would have further poisoned relations between Washington and Beijing. At his news conference with Mr. Abe, Mr. Trump took pains to say he and Mr. Xi were developing their own rapport.


“It was a very, very warm conversation,” Mr. Trump said. “I think we are in the process of getting along very well, and I think that we’ll also be very much of a benefit to Japan.”


Mr. Abe appeared pleased, too, noting that the president had reaffirmed the American security guarantee to Japan, including over what the Japanese call the Senkaku Islands, which are administered by Japan but claimed by both Japan and China. That was a far cry from Mr. Trump’s suggestion during the campaign that the United States might walk away from the alliance.


Michael J. Green, an Asia director in Mr. Bush’s National Security Council, noted that in the weeks before Mr. Trump took office, he suggested that the key pillars of America’s relationship with both China and Japan — the One China policy and the mutual defense treaty — would both be on the table, chips to be used in a broader negotiation.


“What’s been so interesting about the last few days is that Trump took both issues off the table,” Mr. Green said. “You can hear the sigh of relief across the whole Asia-Pacific region.”