When President Trump announced that he would pick Jim Mattis, a retired four-star general, to serve as secretary of Defense, the decision was met with relief. In January 2017, Trump seemed like he might well be able to give conservatives real leadership and credibility on foreign policy. Mattis had the respect of the president and the military as well as the knowledge and experience necessary to navigate emerging threats to the world order largely built by the United States. He was also someone that even those skeptical of Trump thought could steer the administration to make the right choices.

In short, Mattis brought credibility to a White House that desperately needed it.

But Mattis, for all of his appeal to Trump's administration, proved unable to push the president to address key foreign policy realities. Instead, Trump insisted on publicly undermining allies, making a show of support for enemies of the U.S., ignoring intelligence briefings and pursuing foreign policy by tweet.

In the end, Mattis had had enough with an increasingly unreliable president — and the decision to leave Syria at the expense of our allies seems to have been the final straw.

In his letter to Trump, he explained, "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."

When Mattis does leave in February, he will take with him the same credibility he brought — credibility that Trump needs now even more than ever.

For Trump, that is a huge loss with repercussions far beyond foreign policy.

Indeed, over the past few days, fault lines have emerged between key Republicans and the Trump administration.

Senate Majority Leader Mitch McConnell, R-Ky., called the secretary of Defense's departure “disturbing.” Noting, “I am particularly distressed that he is resigning due to sharp differences with the president.”

Sen. Lindsay Graham R-S.C., who proved a key ally during the fight to confirm Kavanaugh, has become a harsh critic decrying the president’s actions in Syria and saying that Mattis' resignation brought him sadness.

Sen. Marco Rubio, R-Fla., also added to the chorus of concern, tweeting, "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."

Although Trump is no stranger to criticism from conservatives, these criticisms are more than disagreements on strategy and questionable rhetoric, and instead represent a deep loss in credibility for the administration.

And Trump, soon to face a barrage of investigations from a Democratic controlled House as well as likely impeachment, will be in need of credibility and supporters.

With Mattis gone, Trump seems to have lost both.

That could well prove fatal to the administration if the loss of credibility means Trump comes up short of the 34 Senate Republicans he would need to prevent the Senate from removing him from office.

Given that predicament, Trump would do well to follow Mattis’ advice as outlined in his resignation letter. Doing so would bolster Republican support and stem some of the damaging flow of criticism from conservatives. It would also demonstrate to those like McConnell who have expressed concerns, that those worries are unfounded.

More importantly, following Mattis' advice would also likely prevent Trump from making even more damaging and rash decisions on foreign policy that would given Republicans fewer reasons to throw their lot in with his should it come to that.