“Let it be an arms race,” Mr. Trump told MSNBC

Trump Makes Foray Onto Obama’s Turf

President-elect wades into foreign policy before taking office, setting up confrontation between outgoing and incoming administrations

By   Carol E. Lee and Peter NichoUpdated Dec. 23, 2016 7:20 p.m. ET

WASHINGTON—President-elect Donald Trump is upending the modern convention that the U.S. speaks with one voice on foreign affairs, plunging into some of the most sensitive national-security matters before he takes office.

Mr. Trump has launched a series of challenges to President Barack Obama’s policies on nuclear weapons, China and the Israeli-Palestinian peace process, setting up a rare and increasingly public confrontation between outgoing and incoming administrations.

Mr. Obama on Friday brushed back pressure from Mr. Trump to block a United Nations Security Council resolution harshly criticizing the expansion of Israeli settlements. Mr. Trump on Thursday called on the administration to veto the resolution. But Mr. Obama instead chose to break from longstanding U.S. policy and allow it to pass.

“There’s one president at a time,” said Ben Rhodes, a deputy national-security adviser. He said the president believes “it’s important that the world understands who is speaking on behalf of the United States until Jan. 20.”

Mr. Trump took to  Twitter after the vote, saying: “As to the U.N., things will be different after Jan. 20th.”

While Mr. Obama’s move suggests it may be difficult to eclipse a sitting president who has said he intends to “run through the tape,” Mr. Trump’s policy pronouncements as president-elect could send mixed signals to America’s allies and partners overseas about who is in charge, experts and analysts said.

Mr. Trump signaled soon after the election that he planned to take a different posture during the transition, when he took a protocol-breaking phone call from the president of Taiwan.

The White House was caught off guard and fielded angry protests from Beijing.

Jon Alterman, a national-security expert at the Center for Strategic and International Studies, said Mr. Trump appears to be using his transition to test the waters on some issues. “It feels to me partly like he’s just thinking out loud trying to imagine what a Trump foreign policy will be,” Mr. Alterman said.

Brian Katulis, a senior fellow focused on national-security issues at the Center for American Progress, a liberal think tank, said Mr. Trump is deviating from a longstanding tradition that ensures “continuity” in foreign policy even when the presidency changes parties.

“What he’s signaled on a number of different fronts since his election demonstrates that he has an unorthodox and unconventional way of dealing with the world,” Mr. Katulis said.

“It’s deeply unsettling to a number of our longstanding partners.”

Ken Duberstein, a chief of staff under Republican President Ronald Reagan, countered, saying Mr. Trump’s approach could pay dividends. The president-elect is “basically signaling to the world the way he will conduct things once he is in fact president,” he said. “It’s reassuring to many of our allies and it is setting the stage for an understanding from our adversaries that there will be a new sheriff in town.”

Mr. Trump has just this week waded into two of the most hot-button foreign-policy issues, both raising the prospect of expanding America’s nuclear arsenal and, at the behest of Israel, pressuring Mr. Obama to veto the U.N. resolution.

Mr. Trump said Friday that he wouldn’t shrink from a nuclear-arms race, doubling down on his tweet a day earlier saying the U.S. needs to expand its nuclear capabilities. “Let it be an arms race,” Mr. Trump told MSNBC “Morning Joe” co-host Mika Brzezinski. “We will outmatch them at every pass and outlast them all.”

Mr. Trump’s top spokesman, Sean Spicer, later sought to play down the interview.

But Mr. Trump’s comments—which followed Russian President Vladimir Putin’s statement that Moscow needs to build up its military, including nuclear weapons—are reigniting concerns among critics that he lacks the temperament for the presidency.

“The words presidents speak or tweet or write can send armies marching and markets tumbling,” said David Axelrod, who was a longtime adviser to Mr. Obama and supported Hillary Clinton’s presidential campaign. “I think there was this hope or expectation that the weight of the presidency or the impending presidency would sober him and this is evidence that that’s not the case.”

Rep. Adam Schiff, the top ranking Democrat on the House Intelligence committee, expressed alarm at the casual tone Mr. Trump is using to discuss major changes to the nation’s defense posture and security policy.

“These opaque, oracular statements that are coming out of his Twitter account are the subject of multiple interpretations and they’re a dangerous thing to do as president-elect,” said
Mr. Schiff. “They could be a potentially catastrophic thing to do as president.”

Also on Friday, Mr. Trump released a letter he received from Mr. Putin, in which the Russian president says he hopes that “we will be able—by acting in a constructive and pragmatic
manner—to take real steps to restore the framework of bilateral cooperation in different areas as well as bring our level of collaboration on the international scene to a qualitatively new level.”

Mr. Trump responded to what he called “a very nice letter from Vladimir Putin” with a statement saying, “His thoughts are so correct. I hope both sides are able to live up to these thoughts, and we do not have to travel an alternate path.”

—Ben Kesling and Byron Tau contributed to this article.