Here's why:
* The Prize: Agamemnon takes Briseis, a beautiful captive woman and a prized possession of Achilles, as his own.
* The Insult: Agamemnon justifies his actions by claiming he has the right to take whatever he wants as the commander. He also belittles Achilles, calling him a "greedy cur" and a "coward".
* The Response: Enraged, Achilles withdraws from battle, refusing to fight for the Greeks. This action has devastating consequences for the Greek army, who are struggling to defeat the Trojans.
This event sets the stage for much of the conflict in *The Iliad*.