President-elect Donald Trump addresses supporters on election night in New York. (Jabin Botsford /The Washington Post)

There have been times in America’s past when an untested new president faced immediate calamity and potentially world-shattering disaster, from the Civil War to the Cuban missile crisis. But the broad array of international challenges now facing Donald Trump is among the most daunting and dangerous in modern U.S. history.

After a campaign of bombastic sound bites and often contradictory policy prescriptions, Trump’s plans remain opaque for dealing with issues including terrorism, Russian aggression and multiple shooting wars in the Middle East.

He has called for increased military strength and more forceful American leadership, while also speaking of stepping back from U.S. responsibilities as the free world’s primary protector. He has invited China to invade North Korea and “solve that problem,” but also said he would host North Korean leader Kim Jong Un at the White House.

He said he would renegotiate the Iran nuclear deal, and then called for strict enforcement of the existing agreement. His plan to combat the Islamic State, Trump said during the campaign, is a secret.

In Mexico, China, Russia and Israel we ask people what they think of the election of Donald J. Trump as the 45th president of the United States. (Jason Aldag/The Washington Post)

With little clarity on much of what he intends to do, the best initial indicator of Trump’s approach may be those he chooses for his national security team. Many of the names floated so far strike fear in the hearts of mainstream Republicans as well as Democrats, while others instill a sense of reassurance.

Perhaps more than any other area of policy, the universe of potential picks for national security Cabinet positions, agency heads, deputies and those below is limited by the large number of Republican establishment figures in the field who said early and often during the campaign that they would never support a Trump candidacy or work in a Trump administration.

Determining who is even on a list of possibilities is hampered by the fact that those who have been organizing the transition apparently have had little contact with Trump or the tightknit circle of family and top campaign officials around him.

There has been “a firewall between people who had access to the man himself and people doing stuff that was serious about a transition,” said one prominent Washington conservative who insisted on anonymity to speak candidly. “The Trump transition to most of us is just a black box.”

Even if some of the senior Republicans who denounced Trump decide to offer their services, he has given no indication he is prepared to forgive and forget. Blackballing job contenders, of course, is not unique to Republicans. Liberal advocacy groups, reportedly with guidance from Sens. Bernie Sanders (I-Vt.) and Elizabeth Warren (D-Mass.), had assembled names unacceptable for positions in a Clinton administration.

On the Republican side, “Trump is like Nixon,” the conservative said. “They keep lists.”

In a post-election message to State Department employees Wednesday, Secretary of State John F. Kerry reminded them of the need “to continue moving ahead with all the activities and projects on which you are currently engaged. The pace of events across the globe does not allow for timeouts.”

The “second imperative,” Kerry said, is “to welcome our incoming colleagues warmly and professionally and to provide them with all the assistance they need to ensure a seamless transition from one administration to the next.”

Trump’s personnel choices could ease or deepen concern in Cabinet departments and agencies such as the CIA, where Trump has said he would reinstate the use of torture and detention for terrorism suspects. Former agency officials said that any attempt to reintroduce harsh interrogation techniques such as waterboarding would meet fierce internal resistance.

Among the rumored candidates for secretary of state, former House speaker Newt Gingrich (R-Ga.) and former U.N. ambassador John Bolton — both outspoken Trump supporters — are viewed as anathema by many current diplomats and as loose cannons even by many of their fellow Republicans. Sen. Bob Corker (R-Tenn.), the chairman of the Senate Foreign Relations Committee, is seen as the more mainstream candidate.

Corker said that he had spoken to Trump and Vice President-elect Mike Pence on Wednesday to offer his congratulations but that he’d had no conversations with them about a position in the new administration. Although he has expressed interest in the State Department job in the past, “it’s way too soon for that kind of thing. . . . These are decisions that others are making,” Corker told the Tennessean newspaper Wednesday.

At the Pentagon, Trump has criticized the counterterrorism policies of current senior officers, saying he would replace them, but also has said he would defer to them on new policies. Among other things, current and former defense officials have expressed unease over threats to withdraw from NATO, use nuclear weapons, and engage in military partnerships with Russia while responding more aggressively to Chinese expansionism.

The possibility that retired Lt. Gen. Mike Flynn, the former head of the Defense Intelligence Agency and an active Trump adviser and surrogate who is highly critical of current policy, would be named as defense secretary or national security adviser does not sit well within the Pentagon or the intelligence community, career officials in both have said. Flynn’s appointment to a Cabinet position would also require a congressional waiver of a law restricting activities of former senior military officers.

Sen. Jeff Sessions (R-Ala.), another senior Trump adviser, is said to be interested in the top Defense Department job. Though a member of the Senate Armed Services Committee, he is much better known for his interest in immigration than the military.

Amid the many swirling rumors — and a dearth of actual information — former CIA director and retired Gen. David Petraeus is said to be in the mix for a top national security job, as are House Homeland Security Committee Chairman Michael McCaul (R-Tex.), former congressman Mike Rogers of Michigan and George W. Bush national security adviser Stephen Hadley.

Trump’s lack of foreign policy experience could also shift more focus back to Congress, where Republicans who have sharply criticized the level of unilateral control President Obama’s White House has exercised over national security — and some of whom have denigrated Trump as uninformed on the issues — are chafing for a bigger voice. At the same time, Trump’s lack of a deep bench to populate the National Security Council staff could result in a reduction from an Obama White House team seen as bloated.

But Trump “wasn’t elected on the strength of his foreign policy,” said Danielle Pletka, vice president for foreign and defense policy studies at the American Enterprise Institute. A non-politician with no record to judge, Trump “said enough to get people to elect him,” Pletka said. But he has also “made a lot of statements that are totally contradictory,” she said. “. . . I have absolutely no idea how he’s going to govern.”

As Trump makes his choices and begins to work through the various policies he has described in bullet points, some issues will require immediate attention. They include the fight against the Islamic State, Syria’s simultaneous civil war and broad Middle East instability; a newly aggressive Russia that U.S. intelligence says has directly intervened in the American electoral process; and the likelihood that North Korea will have a missile-mounted nuclear weapon capable of reaching U.S. shores during the first year of Trump’s presidency.

Others, including China, Iran and global terrorism, are on a larger plate of pressing strategic issues. In Asian capitals such as Tokyo and Seoul, officials voiced trepidation over Trump’s campaign promises to make them pay more for a U.S. security shield. In Damascus and Jerusalem, there was optimism that pressure to make concessions to their opponents would lessen.

At NATO headquarters in Brussels on Wednesday, one ambassador sent an email to another summing up the high state of anxiety given the president-elect’s threat to withdraw from the alliance if other members failed to meet their obligations. The email read, “Oh my god.”

Adam Entous contributed to this report.