Defense Secretary James Mattis will step down from his post in February, President Donald Trump announced on Thursday, bringing an end to yet another cabinet official's tenure since Trump took office two years ago.

Mattis expressed considerable differences with Trump in his resignation letter to the president, writing that he is leaving "because you have a right to have a Secretary of Defense whose views are better aligned with yours."

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The move sent shockwaves through Washington, coming at a time when Republicans have become increasingly frustrated with Trump's military policy decisions, which have ran counter to the advice of many of his top advisers and traditional GOP orthodoxy.

"That’s what happens when you ignore sound military advice," Illinois Rep. Adam Kinzinger, a Republican, wrote on Twitter in his immediate reaction to Trump's announcement.

Sen. Lindsey Graham said in a statement he felt a "great sadness" upon learning of Mattis' resignation, saying he "is one of the great military leaders in American history" and should be proud of his service.

"General Mattis is a combination of intellect and integrity," Graham added. "He has been in the fight against radical Islam for decades and provided sound and ethical military advice to President Trump. He is a role model for the concept of Duty, Honor, Country."

Nebraska Sen. Ben Sasse, a frequent critic of Trump, said in a statement that it is "a sad day."

"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," he added, noting that an isolationist foreign policy would pose a grave threat to the United States.

"Radical jihadists are still at war with us, and NO, MR PRESIDENT, ISIS is not gone," Sasse wrote on Twitter. "It's not true — and just proclaiming it doesn’t make it so."

Democrats characterize Mattis' departure as a crisis

Democratic Sen. Chris Coons said Mattis' resignation "more bad news for our national security" because he was "one of the most seasoned & capable advisors" Trump has had.