Agamemnon inherited the role of king from his father, and his community expects him, as king, to stabilize society, arbitrate disputes, and call council meetings and assemblies. He is also commander-in-chief of the armies. Both Odysseus and old Nestor (two of his commanders) attempt to maintain Agamemnon's authority because they recognize that supporting Agamemnon is the only way to ensure an effective and meaningful policy of order. Agamemnon is, after all, the king and their leader.
Yet despite that Agamemnon is king and has enormous power and social position, he is not necessarily the best qualified for the role. Old Nestor frequently advises Agamemnon because Agamemnon needs counsel. Almost immediately, the reader sees that Agamemnon often allows his over-wrought emotions to govern major, critical decisions. Nestor advises Agamemnon against taking Briseis from Achilles, but Agamemnon doesn't listen, thereby setting up a chain of events that results in the deaths of hundreds of Achaian soldiers.
Unfortunately, Agamemnon was born to a role greater than his ability, and Achilles, another hot-tempered Greek, was born to a role less than his ability. Both men are great men, but both are quick to anger, and both are conscious of the roles that they must play within the heroic code. Neither man is willing to compromise or to accept a seemingly lower status within the heroic code, and so their quarrel over Briseis results in a tragic breach between the two — one that creates a central conflict in the Iliad.
Note, however, that Agamemnon shows devotion to and concern for his brother, Menelaos. Agamemnon realizes that order in the Achaian society depends upon Helen's return to Menelaos. He is aware of the importance of family order if all of society is to remain cohesive. Yet with all these good traits, Agamemnon is plagued with other traits that undermine his good qualities and contribute to self-created problems.
Agamemnon is weak; he vacillates. During periods of depression and discouragement, he makes wrong decisions, and he is sometimes unfair. He fails to realize that a king must not succumb to his own desires and emotions. He does not realize that authority demands responsibility and that his personal wishes must be secondary to the needs of the community. His failure to understand the limitations of power causes him to make his first error: He insists on keeping his Trojan war prize, Chryseis, despite her father's pleas. He likes her, and he believes that he will lose face if he returns her.
Eventually, Agamemnon learns to listen to the counsel of old Nestor, Odysseus, and Diomedes, but it seems clear that his emotional makeup and inability to judge do not fully qualify him for kingship. Even after he finally admits to his madness in dealing with Achilles and attempts to reverse the error with gifts and the return of Briseis, he only insults Achilles. When his courage flags and he becomes depressed, he wants to abandon the Trojan War altogether.
Despite Agamemnon's prowess as a warrior, as a king he too often exhibits the characteristics of stubbornness, cowardice, and immaturity. As the reader carefully studies Agamemnon's character, some growth in understanding can be seen in him particularly in Book IX when he sends the embassy to Achilles. At the end of the epic, Agamemnon is a much greater leader than in the early books, even though he never reaches the same stature as several of the other warriors.