In The Iliad the rage of Achilles takes place over a considerable length of time. This means that, although Achilles is absent from the early action on the battlefield while he sulks in his tent, his brooding presence is never very far away.
Achilles is the best of the Achaean warriors, so his absence from the thick of battle has a disastrous effect on his comrades. Without Achilles fighting alongside them, they're in serious danger of losing the war. But even as his comrades suffer a series of potentially catastrophic reverses, Achilles stubbornly refuses to emerge from his tent, still boiling with anger as he is over Agamemnon's insulting behavior.
On a structural level, Achilles' implacable wrath organizes the poem; almost everything that happens after Achilles retreats to his tent is in some way related to it. We sense that Achilles will return to the fray at some point; we just don't know when or why. But in the meantime, we do know that for as long as he remains out of action, the Achaeans will continue to suffer great losses.
When Achilles finally decides to return to the fray to avenge the death of his beloved Patroclus, it's relatively late on in the poem (book 20 out of 24). This is a deliberate strategy on Homer's part to heighten tension, to make us wait for Achilles' return until we can wait no longer. Yet even when Achilles finally does set foot on the field of battle once more, his rage is still much in evidence, only this time it's directed where it should be: at the enemy Trojans, not at his fellow Achaeans.
So one could argue that The Iliad is organized thematically according to a very broad tripartite structure: the wrath of Achilles; the reaction of mortals and gods to that wrath; and the target at which that wrath is directed. Just about everything that happens in the poem can be brought under one or more of these headings.