The battles continued for nine years. Chrysis, priest of Apollo and father of Cressida, came to Agamemnon and pleaded for the release of his daughter. But Agamemnon was uncompromising and brutally insulted the priest, who prayed to Apollo to avenge this unacceptable behaviour (hubris). Apollo responded by punishing the Achaean military with an exterminating plague. Finally, Agamemnon, in order to get rid of this calamity, and after pressure from the other Achaeans, was forced to return Cressida to her father.
But then he bragged saying that he would take, by force if necessary, any Achaean loot he liked, even Briseis from the tent of Achilles. The two men argued intensely and Achilles reached the limits to attack Agamemnon with his sword. The goddess Athena intervened invisibly to dissuade him.
Achilles finally agreed to hand over Briseis, but felt great dishonor and refused to continue fighting. His wrath (ire) was so great that he begged his mother Thetis to intervene to Zeus, to allow continued success to his enemies, the Trojans.
And that's what the father of Gods did. In a conference among the ranks of the Achaeans they decided to continue the war even without Achilles as the gods led them to believe that the war was near to an end. For the first time since the beginning of the war, both camps had all their allies available.