Donald Trump. AP Republican national-security leaders and experts who signed an open letter opposing Donald Trump in March have reacted to his presidential win with a range of emotions, from cautious optimism to abject terror.

"What I’m thinking is it's the end of the liberal order as we know it from 1945," Daniel Drezner, a professor of international politics at Tufts University, told Business Insider. "Now that might be an overreaction … but that's what it feels like right now.”

Drezner was one of more than 100 Republican national-security experts who, during the GOP primaries, added their names to a letter asserting that Trump was unfit for office and would "make America less safe." Another letter signed by 50 GOP officials warning that he would be "the most reckless president in American history" circulated in August.

Drezner noted a "surge of populist nationalism" in recent years, such as the Brexit vote and the election of Phillippine President Rodrigo Duterte, which ultimately culminated in the election of Trump.

"I have to admit this: My first reaction was, 'I can’t believe this happened," Tom Nichols, a professor at the Naval War College who made clear he was only representing his own personal views, told Business Insider. "But my next reaction has been, 'Of course this happened.' Because Hillary Clinton was such a terrible candidate.”

Now that Trump has secured victory, reports have suggested he could tap advisers such as retired Lt. Gen. Michael Flynn or Alabama Sen. Jeff Sessions to prominent positions within the Pentagon. The developments could very likely lead to a rollback in Obama administration policies, such as sequestration and lifting the ban on women in combat.

And Trump's foreign-policy positions, which have at times swung from isolationism to military adventurism, and his embrace of Russian President Vladimir Putin, have some experts who signed the letter afraid of the unexpected to come after January 20, 2017 — Inauguration Day.

"I'm totally f---ing horrified," Drezner said. "The fact that they get to be back in power is going to be interesting."

What happens next?

A US Army soldier and a member of the Afghan Uniform Police arm wrestle prior to a joint patrol near Command Outpost AJK (short for Azim-Jan-Kariz, a nearby village) in Maiwand District, Kandahar Province, Afghanistan, January 28, 2013. REUTERS/Andrew Burton

"There's a lot of mystery" as to what Trump will do as president, said David Adesnik, a policy director for independent presidential candidate Evan McMullin who signed the March letter. "Because he changes positions radically on so many things."

"We really don't have a good sense of how he feels about US interests abroad," Michael Auslin, director of Japan Studies at the American Enterprise Institute and author of the forthcoming book "The End of the Asian Century," told Business Insider.

Others in the intelligence community echoed those concerns, telling The Washington Post there was a palpable "fear of the unknown."

At least one area on which Adesnik and others hope Trump changes course is his position on the use of torture. On the campaign trail, Trump often endorsed the enhanced-interrogation tactics carried out under the Bush administration, saying the US was going to "have to do things that are unthinkable." He has since offered varying walkbacks of those statements.

"His most egregious comments about ordering people to commit torture," Adesnik said. "Hopefully it’ll never come to that.”

On Wednesday, Republican Sen. Tom Cotton of Arkansas, a former Army officer, told CNN that waterboarding was not torture, perhaps foreshadowing greater Republican support for the now-banned practice.

Trump also owes specifics on his plan to defeat ISIS, since he had repeatedly refused to provide even general themes on the campaign trail about how he would combat the terrorist group. He often used what he called a need for surprise as cover for what many believe was no plan at all.

"There is no secret plan" that Trump had hidden away during the campaign, Drezner said. "I'm sure they'll come up with one now."

"I don't think [the Trump campaign] expected to win," Nichols said.

"This is ‘The Candidate’ where he wins and in that last frame, he turns and says, 'What do we do now?” he added, mentioning a classic 1972 film starring Robert Redford.

Cautious hope

Despite the initial shock of a Trump win among so-called Never Trump Republican national-security experts, some were expressing hope for what could happen next, though even that was somewhat tepid.

"Our job is not to pretend that he didn’t win or not to say that we aren’t going to help him, because he is our president," Auslin said. "A lot of people that wrestled with whether or not to support Trump during the primaries and the campaign ... the calculation changes once he’s the president."

Auslin wouldn't say whom he voted for in the election, but he did say that he would think about serving in a Trump administration if he were asked.

"I would very seriously consider a request to serve in the Trump administration," Auslin said, because "at some point politics ends and policy begins. It’s up to Trump himself to make that transition from politics to policy."

Dr. Peter Mansoor, a retired Army colonel who served as one of the top advisors to Gen. David Petraeus during the Iraq surge, made clear he would not serve a Trump administration in any capacity. Nichols also rejected the idea of joining a Trump administration, though he added that he had no desire to serve any political appointment, regardless of who was in charge.

"I think anyone who signed that letter shouldn't be under any illusions that they'll be offered anything in a Trump administration," said Mansoor, now a professor of military history at Ohio State University.

"Although I have been warning — along with many others — of the catastrophic consequences of a Trump presidency," Max Boot, a senior fellow at the Council on Foreign Relations, wrote in Foreign Policy, "I have no idea what he will actually do. Nobody does, probably including Trump himself."

But, he concluded: "I’m hoping against hope that he will grow in the White House — that the office will make the man. Because if that doesn’t occur, the consequences are too ghastly to contemplate."