James Mattis had history of disagreements with Trump before resignation

Defense Secretary Jim Mattis abruptly announced he will resign after President Trump stunned advisers with this week's decision to pull U.S. troops out of Syria. Sources tell CBS News the president is also ordering 7,000 troops to leave Afghanistan – a position Mattis also opposes.

The retired Marine general wrote in his resignation letter, "You have the right to have a secretary of defense whose views are better aligned with yours." Mattis also mentioned Russia and China in his resignation letter, warning that the U.S. must be resolute with both nations. He delivered that letter personally to the president Thursday afternoon. In it he says the U.S. must remain clear eyed about countries like Russia, China and North Korea, but his most profound disagreement with the president is over the treatment of American allies.

From the start Mr. Trump was a difficult boss to work with. When he announced his travel ban on mostly-Muslim countries, it was the first Mattis had heard of it. He also didn't know Mr. Trump would tweet that transgender service members would no longer be allowed to serve in uniform, reports CBS News correspondent David Martin.

The final straw was the president's decision to pull U.S. troops out of Syria.

The fundamental disagreement was over, as Mattis put it in his letter, "my views on treating allies with respect."

"NATO's hard power and a defensive orientation is our ticket to peace," Mattis said May 2017. Meanwhile, Mr. Trump has consistently lambasted NATO members for not spending enough on defense, saying in October, "What we're doing with NATO, where for many years they're not paying their bills."

Military will look at Mattis resignation with some "shock and dismay," Winnefeld says

"For Secretary Mattis who has never been known to quit anything – every star that he has on his shoulders was earned in the blood and the mud of battle. And that has to be taken into account when he says we are betraying our allies under these circumstances," former Defense Secretary William Cohen said on CNN.

On Thursday White House Press Secretary Sarah Sanders tried to downplay the differences between Mattis and the president.

"He and the president have a good relationship, but sometimes they disagree," Sanders said.

But lawmakers from both parties voiced concerns and criticism about Mattis' resignation.

"Our troops look to Secretary Mattis as a leader, and now, he is going to be leaving them. This is very serious for our country," House Minority Leader Nancy Pelosi said.

Republican Majority Leader Mitch McConnell wrote he was "distressed" by Mattis' departure, saying the U.S. must maintain "a clear-eyed understanding of our friends and foes, and recognize that nations like Russia are among the latter."

Republican Sen. Marco Rubio tweeted that the letter shows "we are headed towards a series of grave policy errors which will endanger our nation... and empower our adversaries."

Mattis said he'll leave on Feb. 28. The White House said Mr. Trump hopes to name his replacement by the end of the year. Despite the tone of Mattis' letter, Mr. Trump has so far avoided criticizing him. Instead in a tweet, he thanked Mattis for his service.