James Mattis doesn’t seem like someone intent on self-glorification.

The secretary of defense appears to be motivated by love of country, devotion to public service and to living by the code instilled in the Marine Corps from which he retired as a four-star general.

However, his letter of resignation has emerged as an instant iconic message that might stand as a defining epitaph — perhaps the defining epitaph — for the presidency of Donald J. Trump.

The closest recent example I can find that resembles the tone Mattis uses in his resignation letter comes from the late Cyrus Vance, who in 1980 resigned as secretary of state to protest President Carter’s decision to launch the ill-fated mission to rescue the American hostages who were held in Iran.

He calls out the president for his lack of understanding of the importance of our alliances, for his failure to understand the consequences of presidential actions or pronouncements. He scolds the president for his failure to be “resolute” in his relations with our nation’s traditional adversaries, such as Russia and China.

He ended the letter without salutation. Simply signing it “James N. Mattis.”

Read the letter here.

Its message should sadden — and frighten — every American who shares his concerns about the presidency and the man who occupies this exalted office.