The theme of the Iliad is in the first word of the poem in Greek, mênis, wrath. The poet does not promise to tell the whole war of Troy, but the wrath of Achilles and what it cost: "the baleful wrath that brought the Achaeans countless griefs." The wrath is born from a question of honor, the prize Agamemnon takes from Achilles, and it spreads to his companions.
Achilles himself describes the wrath in Book XVIII, after losing Patroclus: "wrath, which is far sweeter than dripping honey, and grows like smoke in the chests of men." And in Book XIX he ends it before the army: "now, indeed, I put an end to my wrath, and it is not fitting for me to remain enraged without respite." It ends, however, only after the death of Patroclus, and only then does Achilles stop turning against Agamemnon.
Around the wrath, the poem works through other themes. Death and fate appear in the scale on which Zeus weighs the Kers, the frail condition of men in the image of Glaucus, "as is the generation of leaves, so is that of men," and grief, in Achilles' lament for Patroclus and in Hector's funeral. In Book XXIV, Achilles tells Priam that Zeus keeps on the floor "two jars," "one of evils, the other of goods."
For many readers, the poem moves from wrath to pity: in Book XXIV, the god Apollo says Achilles "has lost pity," and in the same book he returns Hector's body, weeping with the father of his enemy. Others prefer to read the ending as a pause, not an exit, because the war goes on and Achilles' death is foretold. The two ways of reading rest on the text.