The mood at the Pentagon and on Capitol Hill darkened with the resignation of Defense Secretary James Mattis.

A Pentagon official, who spoke on the condition of anonymity on Friday, described the mood inside the building as "eerie."

"Obviously it's close to the holidays so you don't expect to see a lot of people, but there's just a general fear of the unknown," the official said.

The news that Mattis will leave at the end of February came on top of an already heavy plate of turmoil: President Trump's full withdrawal from Syria, the consideration of a drawdown in Afghanistan, a looming government shutdown and tanking stock markets.

But the Mattis exit was the cherry on a sour sundae, given his standing as the most respected member of Trump's Cabinet across the political spectrum.

It left officials and lawmakers in a gloomy mood, worried that Trump's moves in Syria and Afghanistan are just the beginning.

"Imagine a mentor or a friend, someone you thought would be there for the entirety of your tour, someone who everyone thought really highly of, suddenly not there anymore. ... People are really bummed," the Pentagon official said.

Mattis submitted his resignation letter to Trump on Thursday following the president's decision to withdraw all 2,000 U.S. troops from Syria and as he mulls halving the number of troops deployed to Afghanistan.

Trump's special envoy for the global coalition fighting the Islamic State in Iraq and Syria (ISIS), Brett McGurk, also reportedly submitted his resignation on Friday due to his disagreement with the president's decision to withdraw troops from Syria. He was previously planning to leave in February.

Mattis's resignation letter offered a stunning rebuke of Trump's worldview, making clear that Mattis resigned because his views did not "align" with the president's on the value of alliances such as NATO and the anti-ISIS coalition and standing firm against adversaries such as Russia and China.

The letter sent shockwaves throughout Washington, with congressional leaders immediately commenting on how it rattled them.

Rep. Nancy Pelosi (D-Calif.), likely the next House Speaker, said she was "shaken." Meanwhile, Senate Majority Leader Mitch McConnell (R-Ky.), in one of his more critical statements of the president to date, said he was "particularly distressed" by the reasons Mattis resigned.

On Friday, as lawmakers wrangled over ways to avert a government shutdown in mere hours, the grieving over Mattis continued.

Senate Armed Services Committee Chairman James Inhofe (R-Okla.), a staunch Trump supporter, said he was "upset" by the resignation.

"But it wasn't a surprise," Inhofe added. "There was a level of dissatisfaction between the two parties anyway."

Inhofe's Democratic counterpart on the committee, ranking member Jack Reed (D-R.I.), said that he felt "concern for the country" and "concern for our forces in the field" when he learned of Mattis's resignation.

"Someone who is a consummate professional with great judgement has left, and there doesn't seem to be that kind of stability in the White House or advisors," he said.

Andy Keiser, a principal at the lobbying firm Navigators Global who worked on the Trump transition team's national security section, said the strong reaction was driven both by the comfort Mattis himself gave people and by the importance of the Defense secretary job in general.

"I think there's a real fear for what might be next or what might that mean for U.S. policy going forward, so I think that brings an inherent unease," he said.

The somber mood was palpable at the Pentagon on Friday, with a slimmed-down staff prior to the holidays offering no further comment on Mattis's impending departure, or what it would mean for the military in the coming weeks.

Pentagon chief spokeswoman Dana White told The Hill late Thursday that Mattis "summed it up best" in his resignation letter.

"I don't have anything to add," she said.

Sen. Tim Kaine (D-Va.), a member of the Senate Armed Services and Foreign Relations committees, raised the possibility of other Pentagon officials following Mattis.

"I worry about whether his resignation would spark other resignations," Kaine said. "I'm concerned about that. We'll see if that's the case."

The Pentagon official also noted the uncertainty of what would happen to those serving under Mattis within the building.

"What does this mean for the countless appointees who came in with him?" they said. "All the [deputy assistant secretaries of Defense] and [assistant secretaries of Defense] who joined the department only because Mattis was so well respected, what happens to them? ... A lot of uncertainty."

Michael O'Hanlon, a senior fellow in foreign policy at the Brookings Institution, predicted that "folks at the Pentagon will be anxious."

"They've seen worse," he said in an email, adding "not in terms of losing a great boss though."

James Carafano, a defense policy expert at the Heritage Foundation who said it's "wrong and inexcusable" to withdraw from Syria and draw down in Afghanistan without a larger strategy, downplayed the sorrow in D.C., saying "they'll survive."

"They can have all the doom and gloom they want, then leave town for a month," Carafano, who worked on Trump's transition team, said of lawmakers. "They'll be fine. We're not in the middle of an existential crisis."

But senators are bracing for the possibility that the Mattis exit portends greater changes.

"I think what's troubling is that the letter telegraphs potential policy changes like what we saw in Syria," Sen. Marco Rubio (R-Fla.) said, "which I believe embolden our adversaries, undermine our allies and threaten our national security."