Elizabeth D. Samet is the editor of “The Annotated Memoirs of Ulysses S. Grant.”

On the eve of the battle of Agincourt, facing a numerically superior enemy on foreign soil, the eponymous hero of Shakespeare’s “Henry V” circulates incognito among his soldiers to gauge morale. The playwright calls this “a little touch of Harry in the night.” There is a similar scene in former defense secretary Jim Mattis’s “Call Sign Chaos.” As a battalion commander preparing his troops for the Persian Gulf War, Mattis “walked the lines at night” in the Saudi desert before crossing into Kuwait in 1991: “Troops will tell you things when they’re on guard duty in the dark.” He assessed their hopes, anxieties and frustrations, as well as their readiness for the fight, with an eye to whatever material or psychological sustenance he could provide. A little touch of Mattis in the night.

“Call Sign Chaos” is a love story. Whatever else it may be — memoir, leadership handbook, chronicle of two decades of wars gone wrong abroad against the backdrop of a mercenary culture at home riven by “cynicism” and “tribalism” — the book, written with fellow Marine veteran Bing West, always returns to the object of Mattis’s deepest, most sustained affection: the Marine, specifically the infantryman or “grunt,” who risks his life, for his country, yes, but mostly, Mattis maintains, for his brothers-in-arms. “It’s well known among Marines that our greatest honor is fighting alongside our fellow sailors and Marines,” he writes.

Mattis regards himself as their “sentinel.” As almost every page makes clear, infantry troops are ever in his mind’s eye, even after he ascends to the highest echelons, because they are the ones most vulnerable to policy’s potentially lethal impact. Mattis refers to these men as “my lads” or “my young wolves.” Their manhood repeatedly contrasts with what he regards as the enemy’s cowardly lack of it. Women are peripheral to his vision of combat and to the book. An assertion toward the end that the military is “not a petri dish for social experiments” echoes what he has said elsewhere about integrating women into combat units.

Mattis’s explicit motive for writing was to “attempt to pass to young leaders” the lessons of “over four decades of naval service.” One important model is George Washington, whose philosophy Mattis summarizes as “listen, learn, and help, then lead.” Mattis’s core principles would delight many young officers I know: Learn the culture, define the problem, encourage audacity and initiative, underwrite mistakes, protect mavericks, communicate intent clearly.

Readers looking for leadership lessons will find them in abundance, organized into three sections — direct, executive and strategic — and mapped onto Mattis’s career progression. Lessons are delivered in the aphoristic style popular in military and business cultures. Mattis encapsulates the “leadership fundamentals” he learned as a lieutenant in “three Cs”: competence, caring, conviction. He emphasizes “touchstones”: “Attitudes are caught, not taught”; “Trust is the coin of the realm”; and his mantra, a condensation of the Roman general Sulla’s epitaph: “No better friend, no worse enemy.”

This brevity suits well the swift narrative and reflects a military love of inspirational maxims. Yet their formulaic nature seems at odds with the deep thoughtfulness of which this man of action is clearly capable. With messianic zeal, Mattis champions reading as both “honor” and ethical duty. “If you haven’t read hundreds of books, you are functionally illiterate, and you will be incompetent, because your personal experiences alone aren’t broad enough to sustain you. Any commander who claims he is ‘too busy to read’ is going to fill body bags with his troops.”

To meet each new challenge, Mattis turns to his library: Xenophon, Napoleon, Sherman, Grant, T.E. Lawrence and Viscount Slim are among his favorites; he has pored over the campaigns of Alexander the Great; Marcus Aurelius has long been his lodestar. Reading has given Mattis not only a sense of historical continuity but also the agility to change his approach when circumstances demand. Given his earnest, sustained engagement with so many thinkers, it is frustrating to encounter quotations adrift or unexplained, two likely apocryphal Churchillisms, a mischaracterization of Homer’s attitude toward war, an observation on the poet Robert Burns attributed vaguely to “one man” and difficult to source. Even the valiant Marine and gifted writer E.B. Sledge gets short shrift.

Decades of serious reading and combat leadership have shaped what might well be Mattis’s most fiercely held belief: “You don’t always control your circumstances, but you can always control your response.” War is inevitable, chaos endemic to it, but history doesn’t determine “its own unchangeable course,” and men, especially warriors disciplined to a razor’s edge, have the power to shape it: “You are going to write history, my fine young sailors and Marines,” he writes to the 1st Marine Division on the eve of its 2004 deployment to Iraq, “so write it well.”

Through it all, those sailors and Marines remain a point of clarity amid geopolitical turmoil and Mattis’s disappointment at being unable to dissuade the civilian policymakers he swore to obey from ill-advised “strategic gambles.” Mattis wonders whether the military’s “hardwired . . . can-do spirit” diminished the ability of high-ranking commanders to provide frank assessments of strategic feasibility. But the long-term military and political impact of what Mattis laments as “a lack of time to reflect” among those senior military leaders remains an open question in this book. On the Pentagon desk where he signed orders to deploy troops, Mattis placed a card articulating his burden: “Will this commitment contribute sufficiently to the well being of the American people to justify putting our troops in a position to die?” He “would like to think that . . . the answer . . . is ‘yes.’ ”

On his nocturnal ramble though the English camp, Henry V encounters a few alert and prepared professionals; a noisy swaggerer; a melancholy fatalist; and finally a soldier named Williams, who is capable, intelligent, dubious about the king’s cause, yet certain that a lot of soldiers will end up dying for it. When Henry defensively insists that the king’s cause is “just,” Williams replies: “That’s more than we know.”

Call Sign Chaos

Learning to Lead

By Jim Mattis and Bing West

Random House. 300 pp. $28