Here's a breakdown of what happened:
* The Dispute: Agamemnon, the Greek commander, takes Briseis, a captive woman and Achilles' prize of war, from him.
* Achilles' Feeling of Injustice: Achilles believes this is an injustice and a violation of his rightful spoils of war. He feels deeply humiliated and wronged.
* The Root of the Conflict: The real issue at hand is not just the loss of a captive, but the violation of Achilles' honor and his status as the best warrior in the Greek army. He feels disrespected and undervalued.
This insult fuels Achilles' rage and his decision to withdraw from the war, leading to a series of tragic events that form the heart of *The Iliad*.