Fourteen months ago, U.S. Sen. Bob Corker pointed to three men who “help separate our country from chaos.” The Tennessee Republican had in mind Rex Tillerson, the secretary of state, John Kelly, the White House chief of staff, and James Mattis, the defense secretary. Tillerson exited in March. Kelly will be gone soon, and on Thursday, Mattis rattled many in Washington and elsewhere with his letter announcing he would step down in February.

The retired general long has been an exemplary public man, tough, decisive, thoughtful, steady. He appeared particularly key in keeping the chaos at bay, or applying a check to the impulsiveness and poor choices of President Trump. His resignation prompted one former senior administration official to tell the Washington Post: “There’s going to be an intervention.”

Perhaps things will come to such an outcome, enough Republicans rising up to demand better. In reading closely the letter Mattis presented to the president, it is easy to see how events could turn in that direction. The letter serves as a warning to the country, or as that former official added: “He quit because of the madness.”

News accounts have highlighted the recent differences between the secretary and the president involving the choice for the next chairman of the Joint Chiefs of Staff, the sudden decision to withdraw American troops from Syria and then Afghanistan. They have differed on other matters, including the Iran nuclear deal and the value of the military deterrent in South Korea.

The letter goes to something more fundamental in their disagreement. Mattis warns that the president neglects what has served the country so well the past 75 years. He cites his “core belief” that “our strength as a nation is inextricably linked to our unique and comprehensive system of alliances and partnerships.” If the United States “remains the indispensable nation in the free world,” Mattis puts it, “we cannot protect our interests or serve that role effectively without maintaining strong alliances and showing respect to those allies.”

The secretary departs sharply from the stance of a president who often has disparaged allies and even asserted that he sees little, if anything, to be gained in the relationships. What Mattis reminds is that these alliances and partnerships are the American advantage. They make the country stronger, more powerful and influential.

More, Mattis outlines something else fundamental, the nation proving “resolute and unambiguous in our approach to those countries whose strategic interests are increasingly in tension with ours.” He cites Russia and China. He might as well have added that the president has performed Russia’s bidding in clashing with European allies and done the same for China in withdrawing from the Trans-Pacific Partnership.

In his letter, Mattis describes a sound, two-part approach, “treating allies with respect” and “being clear-eyed about both malign actors and strategic competitors.” He then lowers the boom, this view failing to align with the president’s and thus his need to step down.

At one point, Mattis refers to his “over four decades of immersion in these matters.” It is a subtle rebuke to a president who seems perversely pleased with being less informed. It gets to what no doubt has rankled Mattis, the lack of a careful decision-making process, one that taps the related expertise and has benefited him during his career. The mention heightens the devastating critique in his letter -- and the chill at the idea of his departure.

James Mattis isn’t indispensable. What worries is an ill-prepared president failing to see the asset he has, not just in running the Defense Department but in raising the level of the White House game, in the people it attracts and the policy it makes.