Dec. 20 (UPI) -- Defense Secretary James Mattis, who will leave the Pentagon in two months, wrote in a resignation letter Friday that President Donald Trump deserves a military chief "whose views are better aligned with yours."
Mattis has been the only defense secretary of Trump's presidency and will finish more than two years in the post when he retires at the end of February.
"During Jim's tenure, tremendous progress has been made, especially with respect to the purchase of new fighting equipment," Trump said on Twitter. "General Mattis was a great help to me in getting allies and other countries to pay their share of military obligations. A new Secretary of Defense will be named shortly. I greatly thank Jim for his service!"
In a resignation letter released by the Pentagon, Mattis said he's leaving due to disagreements about how the United States should be involved in international issues.
"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," Mattis wrote.
"One core belief I have always held is that our strength as a nation is inextricably linked to the strength of our unique and comprehensive system of alliances and partnerships. While the U.S. 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 he'd been "privileged" to serve as Pentagon chief, which he said "allowed me to serve alongside our men and women of the department in defense of our citizens and ideals." He did not mention Trump by name in the letter.
"It is regrettable that the president must now choose a new secretary of defense," McConnell wrote. "But I urge him to select a leader who shares Secretary Mattis's understanding of these vital principles and his total commitment to America's servicemembers."
Sen. Marco Rubio, R-Fla., a member of the foreign relations and intelligence committees, expressed concern for what he views as a prelude to a series of major policy missteps.
"Just read Gen. Mattis resignation letter. It makes 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," he wrote on Twitter.
Rep. Nancy Pelosi, D-Calif., the front-runner to take over as House speaker next month when Democrats assume control, told reporters she was "shaken" by Mattis' departure, and encouraged all Americans to read Mattis' resignation letter.
"General Mattis was a comfort to many who were concerned about the path the Trump Admin would choose to take," she tweeted. "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."
"Secretary Mattis is firmly in the camp of the job in Syria is not yet done. That abandoning the Kurds now will hurt us down the road," Graham said. "That [the Islamic State] could and probably will come back. And I think that's the universal view of both [Secretary of State Mike] Pompeo and Mattis."
Trump later rebuked Graham on Twitter.
"So hard to believe that Lindsey Graham would be against saving soldier lives & billions of $$$. Why are we fighting for our enemy, Syria, by staying & killing ISIS for them, Russia, Iran & other locals? Time to focus on our Country & bring our youth back home where they belong!" the president tweeted.
Trump has announced several Cabinet departures in recent weeks, including Interior Secretary Ryan Zinke on (Dec. 15), White House chief of staff John Kelly on (Dec. 8), Attorney General Jeff Sessions (Nov. 7) and U.N. Ambassador Nikki Haley (Oct.9).