The novel is set in Vietnam and recounts the dramatic ordeal of a company of the American military, forced first to garrison an outpost in the middle of the jungle, then to abandon it when deemed useless by military command, and finally to retake it in a bloody battle.
Matterhorn is a mountain, a circumscribed territory, a microcosm that allows the author (a highly decorated former Marine) to narrate the absurdity of war and the perverse mechanisms that sustain it.
And he does so from the perspective of the soldiers, all very young.
In war, a soldier is nothing but

the ash of a cruel game...
nothing more than a collection of empty events that will end up as a faded photograph above his parents' mantelpiece.
They too will die, and relatives who do not know whose photograph it was will throw it away

.
In war

the people one loves will die to give meaning and life to what was always thought to be senseless words, a language of death

.
The novel is structured in three parts, which can also be interpreted as a kind of military life path of a young man, still a student: from his initiation as an officer to his forging in the epic struggle against the enemy, down to existential despair and the loss of the only bond that holds value in war: friendship.
Mellas is a nineteen-year-old officer who, as dictated by an absurd military tradition, is sent to the front despite his inexperience.
Mellas must make decisive choices for his own survival and that of the soldiers entrusted to him.
They move through a hostile environment, amid fog, jungle, aggressive insects, and ferocious beasts.
The enemy is invisible, and one often fires without knowing whether anyone is truly there or if a target has been hit.
Yet the young officer is never alone, for the commanders—comfortably safe, drinking and smoking—hound him over the radio, demanding his position, the enemy body count (often fabricated to pacify them), and the reasons why he does not act with greater resolve.

Already weary from lack of sleep, Mellas was now fighting the physical exhaustion of hacking a path through almost impenetrable brush, climbing muddy slopes to gain a ridgeline, to search for a path, a trail.
He was drenched in both sweat and rain.
Strain, Weight, Flies, Cuts.
Vegetation.
He no longer cared where they were or why....
(still) three hundred and ninety-nine days and then stand up and walk away

.
Yet Mellas, too, is seized by ambition, by the desire to advance his career.
He volunteers to go on patrol with a few soldiers.
Deep within the dense tropical forest, they finally encounter a Viet Cong fighter, who is mortally wounded.
He is a young man, just like him and his comrades.
Terrified, he begs for help, but Mellas knows that the only real mercy would be to kill him rather than leave him as prey for the tigers.
He cannot bring himself to do it and returns to the outpost in tears.
He cannot accept that this is what war is, and

in doing so he opened himself up to the pain of seeing thrown away

the bonds tying those slain enemies to those who loved them: mothers, sisters, friends, and wives.
Command has decided that the company must abandon the outpost and reach another location as quickly as possible, still deep in the jungle.
And the Marines are forced into an exhausting march, dragging behind them the wounded and even the corpses of fallen soldiers—one mauled by a tiger, another killed by a strain of malaria found only in that specific region of Vietnam.
Because they failed to reach this new outpost—every bit as useless as the last—within the allotted time, the company is designated as an emergency reserve for the front’s most precarious sectors.
Mellas wants to protest such an unjust decision, but his friend Hawke reminds him that one must play politics—not for one's own sake, since they are merely reserve officers after all, but because command, out of spite, might withhold the helicopters needed to resupply companies at the front and evacuate the wounded and the dead.
The helicopter is a weapon of blackmail! In the novel's second part, Mellas's company is sent to retake Matterhorn, now occupied by the enemy.
Military command does not know how many Vietnamese there are, but it does not matter.
A heroic feat must be performed to cast the regiment’s senior officers in a favorable light and advance their careers.
The company finds itself embroiled in a horrific battle.
It is an epic narrative, a kind of Iliad, complete with its heroes and martial episodes.
What drives the American soldiers is certainly no ideal (why are we here? the soldiers ask themselves; does Vietnam have gold, oil, or some other raw material?).
What moves them to extraordinary acts of valor is brotherhood and rage: saving their comrades, recovering the wounded or at least the dead; fighting with the conviction that their true enemies are the very men who led them into this butchery.
And then there is the taste of killing for killing's sake.
Tension and fear forge a bond between enemies, who are as if bound together in a web of violence and death.
In the third part of the story, Mellas is wounded in combat and consequently sent to convalesce.
Yet upon his return, he discovers that during the lull, the company has been gripped by the racial strife between whites and blacks that plagues American society—a conflict that had temporarily vanished during combat operations.
The solidarity and friendship forged in combat merely suspend social and racial rivalries.
War is not a moment of genuine change; it only habituates one to violence.
Indeed, his friend Hawke is killed in an attack orchestrated by a faction affiliated with the Black brothers.
Nothing truly changes; they are merely shadows,

moving through this landscape of mountains and valleys, changing the character of things as shadows move but leaving nothing changed when they leave.
Only the shadows themselves can change

.
War triggers an individual transformation, not a collective one.

It is undeniably a war book, one in which the epic element prevails.
Yet it would be reductive to view this novel merely as an account of the Vietnam conflict, a sort of prose poem, a modern Iliad.
It is not merely that.
It is an indictment of war.
Narrating it from such a circumscribed space—Matterhorn and its surrounding territory—already allows the reader to enter the mundane yet tragic reality of individual men's experience, shedding the overarching, particularizing frame of the Vietnam conflict.
It speaks of war—of any war.
Then there is the figure of Mellas, undoubtedly autobiographical.
The young man refuses to accept the inexorable laws of military life and war.
Yet he rejects them not out of nostalgia for civilian life, which for him, as for the others, is already considered forfeit.
He refuses to accept the futility of the decisions, the gratuitous suffering inflicted upon the soldiers, and the killing of enemies—unknown to him, yet fellow human beings with lives back home, young men who also had a future.
It is the chronicle of the tragedy of all wars.

The book is very long, meticulously detailed, and at times slips into reportage.
By the end, one grows somewhat weary of so much slaughter.
Yet this is perhaps the author's very intent: to sicken us with war.

Why read it? It is a magnificent war book.