Washington (CNN) North Korea and Russia might not wait until January 20 to test the new president-elect.

The weeks between Election Day and Inauguration Day can be a key time for foreign adversaries to cause mischief for a new administration.

Donald Trump and his transition team must replace 4,000 political appointees right off the bat, construct a foreign policy framework and get up to speed on an exploding array of international challenges.

The staff's change in approach and high learning curve can make it a tempting period for international parties to take steps without US supervision or that aim to disrupt the US government itself.

"Any presidential transition is a very vulnerable time for the country," Director of National Intelligence James Clapper told an audience at the Intelligence and National Security Summit in Washington earlier this fall.

President-elect Barack Obama's 2008 transition period witnessed the deadly terror attacks in Mumbai, India, and an Israeli military incursion into the Gaza Strip.

George H.W. Bush had to deal with the Lockerbie bombing and a deadly aerial clash between US and Libyan warplanes in 1988, while Richard Nixon took over from Lyndon B. Johnson in 1968 with the Vietnam War in full swing.

While these crises weren't necessarily tied to the transition, they kicked off as a new administration with little to no experience on the matter was preparing to take over.

Adversaries look for advantage

"We're also looking at how some of our adversaries might try to take advantage of some type of transition that they think will lead to some discontinuity either in focus or attention," CIA director John Brennan told CNN's Erin Burnett last month.

"The new team coming in, they need to get up to speed very quickly. So right after Election Day, we're going to have the responsibility for briefing that new team, that new president, to make sure that they understand what lies ahead," he added.

The CIA expected its Cold War adversaries to take advantage of the unusually sudden transition between Nixon and Gerald Ford upon the former's resignation. Analysts later wrote in the President's Daily Brief that, "It may be that many have not had time to consider how the situation might be turned to advantage" after America's enemies appeared to not act immediately after Nixon's.

Former senior officials who have been through the presidential transition tend to agree with Brennan's assessment that it's a sensitive time.

"Literally within hours of taking over power, you could be in the middle of a major crisis," Stephen Hadley, President George W. Bush's national security adviser, said at an event hosted by the Partnership for Public Service, an organization that has been pushing reforms to the transition process ahead of Election Day.

Appearing alongside Hadley, Clapper's predecessor, John Negroponte, agreed, stressing the need to adequately brief any incoming president.

"You've got to prepare the president-elect for these potential contingencies, so get them thinking about that they'd do now before it actually happens."

He added, "What are you going to do if North Korea detonates a bomb or China captures all the Spratly Islands?"

November surprises?

One crisis likely to face the incoming president on Day One is the fight against ISIS.

The battle to recapture Mosul, the last ISIS bastion in Iraq, kicked off earlier in the month and is expected to be fierce, with an estimated 4,000-strong ISIS force defending Iraq's second-largest city. The potential for a mass refugee exodus as the assault gets further underway is also growing.

Some analysts also believe that some of America's adversaries are ramping up their activity in order to take advantage of the transition. North Korea, for instance.

The Center for Strategic and International Studies, a Washington-based think tank, released a study this month October estimating that a North Korean nuclear provocation was more likely in the period immediately before and after a US presidential election.

"North Korean missile/nuclear tests and other major kinetic provocations have clustered increasingly closer to US elections," the report said.

Some experts think the Russian and Syrian regimes' intensified bombing of Aleppo, Syria, is part of a bid to recapture that major city before a new administration is inaugurated and able to change US policy towards the five-year-long Syrian civil war -- making a final bloody assault on the city during the transition period likely.

Asked if he thinks Russian President Vladimir Putin might take advantage of the transition, retired Army Lt. Gen. Mark Hertling told CNN: "I don't think he will; I know he will."

Preparations underway

The Trump transition is headed by New Jersey Gov. Chris Christie. The team has long had planners at work, with intelligence briefings having been provided during the campaign and now being stepped up.

President Harry S. Truman inaugurated the pre-Election Day intelligence briefings for nominees after feeling unprepared upon assuming the presidency in 1945 following the death of President Franklin Delano Roosevelt.

"I'm really glad as a citizen he made the generous decision to prepare his successor," Clapper said of Truman in September.

With the election complete, the intelligence briefers will start holding daily briefings with the president-elect, including about the "crown jewels" -- their term for the most sensitive intelligence.

"The sooner these kinds of briefings of the president, presidential candidates, and/or the president-elect start the better and the more deeply they get into it the better, because a lot of these issues are pretty complicated, there's a lot of background to learn," Negroponte said.

"A good time to be asking lots of questions is when you haven't yet assumed office, because the pace will pick up once you get in," he added.

Both Hadley and Negroponte, who were involved in several presidential transitions, said the process has improved significantly in recent years.

"We've come a long way," Hadley said, noting that when he left at the end of Ford administration, all the paper records were taken from his office the day the presidency was handed to Jimmy Carter, making it even more difficult for the incoming staffers to get up to speed.

New laws improve transition

Recent legislation has helped to improve the process, however.

A law passed in 2010 allowing staff picked by the nominees to undergo security clearance investigations before the vote has been held so that they can be ready to start the job on Day One.

And a measure passed this year created presidential transition councils, which can meet with presidential candidates and enable additional planning even before the contenders have secured their party's nominations.

Secretary of the Army Eric Fanning in late October outlined some of the steps the services were taking in order to better prepare the incoming administration.

"Travel dries up right after the election, so that we can make the transition as smooth as possible. So we have a whole series of binders and information papers and information available for the team when it shows up," Fanning said as part of a panel moderated by CNN's Barbara Starr at the Center for a New American Security.

Fanning's Navy counterpart, Secretary Ray Mabus, concurred, stressing the need for a smooth transition.

"One thing the Pentagon does very well in my experience is get you ready to go in in terms of briefings, in terms of where we are in any specific thing, in terms of making sure that nothing falls through the cracks as you move from one administration to the next," he said.

"There's no luxury of having a couple of days after the inaugural to figure things out," he added. "You've got to be ready at 12:01 on January 20 to meet whatever comes."