Republican and Democratic lawmakers expressed concern about the, who had been a stabilizing force in the tumultuous Trump administration. Mattis delivered his resignation letter to President Trump on Thursday afternoon, the day after the president suddenly announced a withdrawal of troops from Syria. Mattis will retire at the end of February.
Mr. Trump's decision to pull out of Syria surprised many Republicans in Congress and in his own administration. Sen. Lindsey Graham, R-S.C., who has become a staunch ally of the president, sharply criticized the decision to leave Syria. Graham said on Twitter that he learned of Mattis' resignation "with great sadness." Earlier in the day, Graham said in a press conference that Mattis had told him that he "doesn't think it's the right time" to leave Syria.
John Kasich, the retiring governor of Ohio who may decide to launch a presidential bid against Mr. Trump in 2020, obliquely criticized Mr. Trump on Twitter, alluding to Mattis' resignation.
"This chaos, both foreign and domestic, is putting America in danger and must stop immediately," Kasich wrote.
Occasional Trump critic Sen. Ben Sasse, R-Neb., said on Twitter that "General Mattis was giving advice POTUS needs to hear."
"Mattis rightly believes that Russia & China are adversaries, and that we are at war with jihadists across the globe who plot to kill Americans," Sasse wrote. "Radical jihadists are still at war with us, and NO, MR PRESIDENT, ISIS is not gone. It's not true — and just proclaiming it doesn't make it so," he added, referring to Mr. Trump's assertions that the terrorist group ISIS has been defeated.
House Homeland Security Committee Chairman Mike McCaul, R-Tex., told reporters that he "slept better at night knowing (Mattis) was in charge of our military." Rep. Adam Kinzinger, R-Ill., tweeted that Mattis' departure is "what happens when you ignore sound military advice."
Presumed incoming House Speaker Nancy Pelosi, D-Ca., told reporters that she was "sad" and "shaken" by Mattis' departure. She said that he had written a "beautiful letter" resigning from office. Mattis' resignation letter was subtly critical of Mr. Trump's approach to foreign policy.
Sen. Marco Rubio, R-Fla., initially wrote on Twitter that he hoped "his decision to resign was motivated solely by a desire to enjoy a well deserved retirement." But after reading Mattis' resignation letter, Rubio tweeted again, saying that Mattis' letter made it "abundantly clear that we are headed towards a series of grave policy errors which will endanger our nation, damage our alliances & empower our adversaries."
Senate Intelligence Committee Ranking Member Mark Warner, D-Va., on Twitter called Mattis' departure was "scary."
"This is scary. Secretary Mattis has been an island of stability amidst the chaos of the Trump administration," Warner wrote. Sen. Richard Blumenthal, D-Conn., called Mattis' resignation "virtually our worst nightmare" which leaves a "chasm of leadership that could be destabilizing around the world."
At least some former national security officials appeared to agree with the more alarmed responses. Gen. Stanley McChrystal, the former commander of U.S. forces in Afghanistan, told CNN's Jim Sciutto that Mattis' departure should concern the public.
"The kind of leadership that causes a dedicated patriot like Jim Mattis to leave should give pause to every American," McChrystal told Sciutto.