President-elect Donald Trump talks to members of the media after a meeting with military leadership at the Mar-a-Lago club in Palm Beach, Fla., Wednesday. (Jabin Botsford/The Washington Post)

Donald Trump’s sudden embrace this week of a nuclear arms race — and his staff’s scramble to minimize the fallout — underscored an emerging modus operandi for the president-elect: governance by chaos.

Since winning the election, Trump has seemed to revel in tossing firecrackers in all directions, often using Twitter to offer brief but provocative pronouncements on foreign and domestic policies alike — and leaving it to others to flesh out his true intentions.

In the past week alone, Trump has publicly pitted two military contractors against each other, sowed confusion about the scope of his proposed ban on foreign Muslims, and needled China after its seizure of a U.S. underwater drone.

But nothing has created more consternation for many foreign policy experts than Trump’s assertion Thursday on Twitter that the country should “greatly strengthen and expand” its nuclear capability.

On Friday, after his staff had tried to temper his comments, Trump doubled down — telling a television talk-show host that in an arms race against any competitor, the United States would “outmatch them at every pass.”

(Peter Stevenson/The Washington Post)

Trump has pledged to shake up both Washington and the world order, and boosters argue that a degree of unpredictability can be useful, particularly when it comes to foreign policy. But the mixed messages and erratic nature of his pronouncements have alarmed even some Republicans, who say it’s important to know how seriously to take the leader of the free world.

“We’re just operating in this world where you cannot believe the things he says,” said Eliot Cohen, a foreign policy expert and former George W. Bush administration official at the State Department. “It will have large consequences for our allies and our adversaries, and it’s going to greatly magnify the danger of miscalculation by all kinds of people.”

[Trump’s latest remarks stoke talk of a nuclear arms race]

Trump’s team has struggled with the new resonance that becoming president-elect has given Trump’s Twitter habit. They have repeatedly said that his statements on social media do not necessarily reflect his official policy and have at times sought to play down the import of his actions.

But Trump supporters say the rest of Washington is going to have to get used to his more freewheeling style.

“People who expect the past are going to be shocked that there’s a new way of doing things,” said Barry Bennett, a Republican strategist who served as an adviser to Trump during the general-election campaign. “This is a glimpse of where he’s headed, and in a way, it’s highly transparent, but just not the way Washington has done business for the past 40 or 50 years.”

But others warn that Democrats and Republicans alike on Capitol Hill could have a hard time reading Trump and discerning his true priorities if he continues to operate as he has during the transition.

“It’s difficult to separate the wheat from the chaff — what comes out that he really wants and what’s just said at the spur of the moment,” said Jim Manley, a former longtime aide to Senate Minority Leader Harry M. Reid (D-Nev.). “In the past, a president-elect doing something like this would have been unfathomable.”

The imbroglio over nuclear arms began Thursday with a tweet from Trump in which he said the United States “must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes.”

Trump made no mention of what spurred his thoughts, but the tweet followed an address by Russian President Vladi­mir Putin in which he said his country’s nuclear stockpile needs fortifying.

Trump’s tweet seemed to signal a break with four decades of policy charted by presidents of both parties — and it sent his staff scrambling to explain his thinking.

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In a television interview Thursday night, Kellyanne Conway — named counselor to the president earlier in the day — played down its sweep.

“He’s not trying to change a policy through Twitter,” Conway told MSNBC’s Rachel Maddow. “What he’s merely saying is that he wants us to be ready to defend ourselves.”

Conway said Trump’s tweet was not necessarily aimed at Russia but directed at “a regime that would do us harm or a rogue nation.”

Trump was not backing down Friday morning, however. On MSNBC’s “Morning Joe,” co-host Mika Brzezinski relayed a conversation she had with Trump in which he reportedly said, “Let it be an arms race. We will outmatch them at every pass.”

As they discussed Trump’s nuclear vow, Brzezinski and host Joe Scarborough, a former congressman from Florida, were both dressed in pajamas as they sat in front of a fireplace on their final show before Christmas.

Shortly after the segment aired, Sean Spicer, who was named Trump’s White House press secretary on Thursday, suggested that Trump was describing a hypothetical situation and speaking about what would happen if other countries don’t “come to their senses.”

“If another country wants to expand their nuclear capability, the U.S. is not going to sit back and idly by,” Spicer said on NBC’s “Today” show. “But just to be clear: The president isn’t saying we’re going to do this. He said unless they come to their senses. It’s a warning to them that this president’s going to take action.”

Democrats, meanwhile, chided Trump for being cavalier about a topic as sensitive as nuclear weapons.

Taking to Twitter himself, Sen. Bernie Sanders (I-Vt.) wrote Friday afternoon that “Congress must not allow the Tweeter in Chief to unleash a dangerous and costly nuclear arms race.”

Rep. Adam Schiff (D-Calif.) tweeted: “Dear Donald Trump. You’re new to this so here’s a list of things to tweet about instead of nuclear weapons. 1.) Literally anything.”

In an interview, Schiff said that in his view, “this just isn’t a way to conduct business, certainly not as president or president-elect. It’s dangerous.”

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Doug Wilson, a former assistant secretary of defense for public affairs in the Obama administration, said it remained unclear to him and many others in the national security establishment what Trump was trying to convey.

“I’m not sure if he’s talking about expanding the nuclear arsenal or he’s mistaken that for modernizing the nuclear force,” Wilson said.

Wilson said that there can be foreign policy advantages to being unpredictable but that “there is a difference in being strategically unpredictable and foolishly unpredictable.” He said it’s too early to know in which camp to place Trump.

On a range of other issues during the campaign and in recent weeks, Trump has made statements that his aides have immediately walked back or softened.

In a brief appearance before reporters this week, Trump seemed to suggest that two of his most controversial campaign proposals — to ban foreign Muslims from entering the United States and to register those who are here — had been vindicated by recent terrorist attacks in Europe.

Later, Conway insisted that a ban on Muslims was no longer Trump’s plan.

After meeting this week with the chief executives of two of the largest defense contractors, Boeing and Lockheed Martin, Trump announced on Twitter that he had asked Boeing to price out the cost of an F-18 fighter jet — which could replace the F-35 jet developed by Lockheed Martin due to cost overruns.

The tweet sent Boeing’s stock up and Lockheed Martin’s stock plummeting — shaving about $1.2 billion off the company’s value.

Some conservatives praised Trump for using his platform to pursue cost savings for taxpayers.

But on foreign affairs in particular, experts warned that Twitter is not the best venue to pursue new policy — even if the aim is disruption.

Thomas Nichols, a U.S. Naval War College professor who is currently writing a book on U.S. nuclear policy, noted that Trump and his staff have now offered multiple explanations for what he meant in his nuclear weapons tweet.

“It’s worse than not having one explanation,” said Nichols, who said he was speaking in his personal capacity. “If you’re going to change policy, then that requires a kind of steely consistency and a lot of disciplined messaging.”

“We’re all spending a lot of time,” he added, “trying to devise the future of America’s nuclear policy out of 140 characters.”