The Greeks learn that kidnapping the daughter of a priest to a god of plague is maybe not the best decision. But if Agamemnon lets her go, then how is everyone supposed to know that he is the best king who deserves a prize? So Agamemnon decides that if he has to let Chryseis go, then he'll have Achilleus' prize, Briseis. Which Achilleus does not appreciate. And he is also related to the gods. This will all be handled maturely and with no bloodshed whatsoever.