Compelled by an oracle to give up HIS war prize (a woman named Chryseis), Agammenon - as King - had the power to take Briseis from Achilles. He did precisely that. Reacting badly to this interference in his life, a sulking Achilles refused to fight against the Trojans.
Knowing the Greeks could not win without the help of their greatest warrior, Agamemnon tried to convince Achilles to set aside their personal differences. Achilles refused, although he reluctantly allowed Patroclus (wearing the armor of his cousin) to fight in his place.
Patroclus, however, was inexperienced and unable to defend himself against Hector, leader of the Trojan forces. The following day, initially believing that the man wearing Achilles’ armor was, indeed, Achilles, Hector killed Patroclus and kept the armor for himself. Achilles, both enraged and grief-stricken over the loss of his friend, agreed to actively engage the Trojans - especially Hector. Before that encounter took place, however, Agamemnon returned Briseis, untouched and unharmed, to Achilles.