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.