Why and how did Apollo attack the Greek?

In the tenth and final year of the Trojan War, the fighting is temporarily stalemated. While on a previous raid, Agamemnon, commander-in-chief of the Achaean forces, has taken as plunder the beautiful girl Chryseis. Chryseis' father, Chryses, is a priest of the god Apollo. Chryses pays a visit to Agamemnon, treating him with great respect and courtesy and offering an opulent ransom in exchange for the freedom of his daughter. Although the Achaeans cry out their approval for Chryses' request, Agamemnon refuses to grant it. He threatens to kill Chryses if the priest should ever come into Agamemnon's presence again. Chryses flees, but he prays to Apollo for vengeance and justice. The god, angered by Agamemnon's disrespect for his priest, rains arrows on the Achaeans. The result is a horrible plague, as men and animals die mysteriously for nine days.