The story of Homer's Iliad actually centers around the "rage of Achilles, contrary to popular belief. At first thought or reading the epic poem seems like its main theme is utlimately the totality and gruesomness of war. In reality it is an ancient "Saving Private Ryan" in that it tells us of the raw details of war without any lack of description and information. However this ultimate devistation and emotion of the actual fighting and Trojan War is not the main focal point of this heroic tale. The real story centers on one awesome fighter, man, and hero--that man is Achilles.
Achilles shows the greatest military prowess of any of the Achaean ranks and has the greatest fighting ability of all of the warriors, Trojan or Achaean. At the beginning of Homer's tale Achilles becomes disenfranchised from the rest of his fellow warriors and chooses to retreat back to his own ships of Myrmidons and refuses to fight for the Achaean cause against the Trojans.
Not only is Achilles plauged and troubled by problems with his fellow soldiers, but he ultimately must face the fact that he has chosen to live a short and glorious life, as opposed to his other option of a long and blase life. Achilles knows that he must ultimately die in the heat of battle and gain great fame for doing so--if he actually existed and the story is true in this respect then he certainly has ascertained immortal fame in the pages of Homer's tale.
Achilles eventually returns to fight on the side of the Achaeans, but not because of anything Agamemnon offers to him in order to get him to return to the fighting. Achilles' best friend and essential "soul-mate", Patroclus, is slain at the hands of the mighty Hector of Troy.
Achilles is hence distraught at this happening and therefore goes to wreak his own havoc with the life of Hector to gain revenge. He manages to eventually kill many Trojans and then finally after chasing Hector several times around the city of Troy, slays him and desecrates his body by dragging him excessively as somewhat of an artifice to get his stored up hate, anger, and fear out.