The day before Secretary of Defense Jim Mattis resigned, President Trump made a sudden announcement: the Islamic State was “defeated” and all U.S. troops in Syria would be withdrawn.

The statement did not sit well with Mattis, who called then-White House chief of staff John Kelly to meet with Trump, The Atlantic reports. The retired four-star Marine Corps general urged Trump to keep U.S. troops in Syria during that meeting on Dec. 20.

But Trump wouldn’t budge and that was the final straw for Mattis.

“You’re going to have to get the next secretary of defense to lose to ISIS. I’m not going to do it,” Mattis told Trump, according to The Atlantic. The interview is the second media appearance for Mattis in two days that sheds light on his tenuous relationship with Trump. On Wednesday, an essay excerpted from Mattis’ new book appeared in the Wall Street Journal.

“When my concrete solutions and strategic advice, especially keeping faith with our allies, no longer resonated, it was time to resign, despite the limitless joy I felt serving alongside our troops in defense of our Constitution,” he wrote in that piece.

In Mattis’ resignation letter, he alluded to the fact that his resignation was due to differences with Trump on key principles. He claimed that Trump deserved to have a secretary of defense whose positions were better aligned with his, and appeared to be critical of Trump’s treatment toward allies.

"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 his resignation letter.

Since Mattis’ departure from the Pentagon, he has returned to the Hoover Institution at Stanford University as a Davies Family Distinguished Fellow and is poised to release a new book about leadership.

“I had no choice but to leave,” Mattis told The Atlantic. “That’s why the letter is in the book. I want people to understand why I couldn’t stay. I’ve been informed by four decades of experience, and I just couldn’t connect the dots anymore.”

Trump has made several disparaging comments about Mattis since he parted ways with the Trump administration, including that Mattis left the job because he was “not too good” leading the Pentagon.

Eventually, the Trump administration backed off a complete withdrawal of U.S. troops from Syria — months after Mattis resigned. According to media reports from February, the Trump administration has kept several hundred U.S. troops in Syria.

Mattis is currently promoting his book, “Call Sign Chaos: Learning to Lead,” which is slated for release on Sept. 3.