Jim Mattis has resigned on principle as Pentagon chief after President Donald Trump decided to slash U.S. troop presence in Syria and Afghanistan, citing in a letter the need for “a Secretary of Defense whose views are better aligned” with the president’s.

Trump announced via Twitter that Mattis, a former four-star general in the US Marine Corps, would be retiring “with distinction” at the end of February.

“General Jim Mattis will be retiring, with distinction, at the end of February, after having served my Administration as Secretary of Defense for the past two years,” the president tweeted.

“During Jim’s tenure, tremendous progress has been made, especially with respect to the purchase of new fighting equipment,” Trump added. “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 the resignation letter (read it below), Mattis praised Trump administration accomplishments, including “improving the readiness and lethality in our forces,” and told Trump that he would remain at the Pentagon until Feb. 28.

But Mattis also suggested his decision was based on differences with Trump, and he even appeared to directly criticize the  president, suggesting he concurred with Trump antagonists who claim Trump has treated allies shabbily.

“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,” Mattis wrote in his resignation letter. “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.

“My views on treating allies with respect and also being clear-eyed about both malign actors and strategic competitors are strongly held and informed by over four decades of immersion in these issues,” Mattis wrote. “We must do everything possible to advance an international order that is most conductive to our security, prosperity and values, and we are strengthened in this effort by the solidarity of our alliances.”

The announcement comes a day after Trump claimed that ISIS had been “defeated,” and after the Pentagon announced that troops were being withdrawn from Syria. The Pentagon directly contradicted Trump the same day, acknowledging that efforts to eliminate ISIS had not concluded.

Earlier on Thursday, just before Mattis’ resignation became public, it was also reported that Trump was considering a massive withdrawal of the approximately 14,000 troops in Afghanistan as well. Some reports suggest this and the Syria decision helped prompt Mattis to quit.

“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,” Mattis said in the letter.

Mattis has frequently dismissed media reports suggesting that his days at the Pentagon would be limited. For example, Mattis indicated to reporters in September that he was not considering leaving his post, and instead said, “I think about doing my job each day.”

“Of course I don’t think about leaving. I love it here. I’m thinking about retiring right here. I’ll get a little place down on the Potomac,” Mattis said.

Trump and Mattis were once close but have reportedly grown distant in recent months, meeting less frequently than they had earlier in his term.

The two have clashed on several issues. Trump reportedly specifically overruled Mattis with his decision to withdraw troops from Syria.

In an October interview on CBS’ “60 Minutes,” Trump labeled Mattis “sort of a Democrat.”