Song of the Wrath of Achilles
In the very first line of the Iliad, the poet addresses the Muse, who inspires him with song, and asks her to sing (through him) the story of the wrath of the son of Peleus, aka Achilles. Achilles is angry with King Agamemnon for reasons shortly to be divulged, but first, the poet lays blame at the feet of Achilles for the death of many of the Achaean warriors. (Homer refers to the Greeks as 'Achaeans' or 'Argives' or 'Danaans', but we call them 'Greeks', so I'll use the term 'Greek' throughout.) The poet then also blames the son of Zeus and Leto, aka Apollo, who has sent a plague to kill the Greeks. (The parallel blame of gods and mortals is common throughout the Iliad.)
Apollo the Mouse God
Before returning to the wrath of Achilles, the poet elaborates Apollo's motives for killing Greeks. Agamemnon holds the daughter of Apollo's priest Chryses (Chryseis). Chryses is willing to forgive and even bless Agamemnon's ventures, if Agamemnon will return Chryses' daughter, but instead, the haughty King Agamemnon sends Chryses packing.
To repay the indignity Chryses has suffered, Apollo, the mouse god, rains arrows of plague on the Greek forces for 9 days. (Rodents do spread plague, so the association between a divine mouse function and delivering plague makes sense, even if the Greeks weren't completely aware of the connection.) The Greeks don't know why Apollo is angry, so Achilles persuades them to consult the seer Calchas, which they do. Calchas reveals Agamemnon's responsibility. He adds that the plague will only lift if the dishonor is amended: Chryses' daughter must be freely restored to her father, and appropriate offerings made to Apollo.
Trade of Briseis
Agamemnon is not pleased with the prophecy, but realizes he must comply, so he agrees, conditionally: Achilles must hand over to Agamemnon Briseis. Achilles had received Briseis as a war prize from the sack of Thebe, a city in Cilicia, where...