President Donald Trump also announced Mattis’ resignation on Twitter, saying he’s “retiring with distinction” and that a replacement will be announced soon.
In a pointed resignation letter, Mattis said his different views with Trump on national security and foreign policy issues prompted his departure.
>> Jamie Dupree : At odds with Trump over Syria withdrawal, Mattis quits as defense secretary
“Because you have the right to have a Secretary of Defense whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position."
"While the US remains the indispensable nation in the free world, we cannot protect our interests or serve that role effectively without maintaining strong alliances and showing respect to those allies," Mattis said.
Mattis’ resignation follows a decision by Trump this week to withdraw troops from Syria, something that members of Congress and the Pentagon have expressed opposition to.
Mattis’ upcoming departure also follows the recent announcement that Chief of Staff John Kelly is leaving his post at the end of the month.
Lawmakers are reacting to Mattis’ decision to call it quits.
House Minority Leader Nancy Pelosi said she’s worried about Mattis’ decision and how it might impact national security.
“General Mattis was a comfort to many who were concerned about the path the Trump Admin would choose to take. His resignation letter is defined by statements of principle — principles that drove him to leave the Administration. All of us should be concerned at this time,” Pelosi wrote in a tweet.
Senate Majority Leader Mitch McConnell issued a statement on Mattis’ announcement also saying he’s worried about his departure.
“I believe it is essential that the United States maintain and strengthen the post-World War II alliances that have been carefully built by leaders in both parties. We must also maintain a clear-eyed understanding of our friends and foes, and recognize that Russia is among the latter, McConnell said.
He also said he was “sorry” to see Mattis go.
“I am particularly distressed that he is resigning due to sharp differences with the president on these and other key aspects of America’s global leadership.”
He encouraged Trump to replace Mattis with someone who is equally qualified to be defense secretary.
Democrat Mark Warner, the top Democrat on the Senate Intelligence Committee, called his impending departure “scary.”
"Secretary Mattis has been an island of stability amidst the chaos of the Trump administration," Warner said, according to CNN.
Trump supporter Sen. Lindsey Graham, who disagrees with Trump’s decision to withdraw U.S. troops from Syria, said he feels a “great sadness” over Mattis’ resignation.
"He is one of the great military leaders in American history. He should be proud of the service he has rendered to President Trump and our nation," Graham tweeted.
In the past year, major players in the Trump administration have been fired or resigned, including former Secretary of State Rex Tillerson, former Attorney Gen. Jeff Sessions, U.N. Ambassador Nikki Haley, former National Security Adviser H.R. McMaster, former EPA chief Scott Pruitt and Chief of Staff John Kelly.