The Iliad announces its subject in its first word, and the subject is not the Trojan war. It is the anger of Achilles, and the poem covers a few weeks near the end of a ten year siege in which that anger does its work. Agamemnon takes a captive woman awarded to Achilles as a prize of honour; Achilles withdraws from the fighting and asks his mother to have Zeus give the Greeks the worst of it, so that they will learn what he is worth; and men die in large numbers because of it, including, eventually, the one he cared about.
What makes him more than a sulking champion is that he is the only figure in the poem who knows the terms. He has been told he can go home and live a long life that nobody will remember, or stay and die young with undying fame, and he has chosen. When ambassadors come to buy him back with gifts he says the thing no other hero says: that cattle and tripods can be won back but a life cannot, and that he is not sure the exchange is worth it. It is the closest the Iliad comes to putting its own value system in question, and it is put by the man with most to gain from it.
He returns to the fighting only when Patroclus is killed wearing his armour, and the grief that drives him then is not about honour at all. What follows is the killing of Hector, the abuse of the body, and finally the scene with Priam that ends the poem.