The incoming president appears to be a profoundly ignorant man who often seems to act on gut impulse or on what pleases the crowd. That is a dangerous combination to have in the White House. Having known General Mattis for many years, I am confident that he will be a restraint on Mr. Trump’s impulsiveness. I also think he will provide a strong counterweight to some of those around Mr. Trump who hold isolationist or pro-Putin views.

He is an unusually forthright man, which is one reason his sayings have become so popular among Marines. (One of the more printable is, “Be polite, be professional, but have a plan to kill everybody you meet.”) He will tell the president what he thinks, and that is a good thing. He has demonstrated repeatedly that he believes it part of his duty to give his candid views to his superiors…

I also think that General Mattis will provide a useful balance to Michael Flynn, the retired Army lieutenant general chosen by Mr. Trump to be national security adviser. General Flynn strikes me as an erratic figure. For example, his joining in the “lock her up” chant at the Republican convention was unseemly and, for a career military officer, unprofessional. Also, General Flynn did not have a good reputation as an administrator when he ran the Defense Intelligence Agency before being moved out by the Obama administration. It will help that General Mattis retired with four stars, while General Flynn wore just three — among military men, rank matters, even in retirement.