In Book II, Agamemnon is sent a false dream by Zeus where he is encouraged by Nestor to gather all his men together and launch an attack on the city walls. What happens, however, is that after gathering his men, as the dream suggested, Agamemnon decides to test the loyalty of his men by saying that he has decided to give up the war and return to Greece. Unfortunately, his men leave straight away, heading for the ships. Note how this is described in the text:
With these words he moved the hearts of the multitude, so many of them as knew not the cunning counsel of Agamemnon. They surged to and fro like the waves of the Icarian Sea, when the east and south winds break from heaven's clouds to lash them; or as when the west wind sweeps over a field of corn and the ears bow beneath the blast, even so were they swayed as they flew with loud cries towards the ships, and the dust from under their feet rose heavenward. They cheered each other on to draw the ships into the sea; they cleared the channels in front of them; they began taking away the stays from underneath them, and the welkin rang with their glad cries, so eager were they to return.
This represents a huge blow to the authority of Agamemnon, as he was obviously hoping for a very different response when he tested his men's loyalty. He is only able to regain his authority and some standing after Odysseus has rallied his troops and Nestor encourages Agamemnon to organise the troops based on their clan and family, so that they can fight next to their friends and family and therefore be inspired. Having suffered such a blow to his authority, this is a very popular move, and it allows the poet to catalogue the heroes that gathered to face Troy and the numbers of the forces that Agamemnon was in charge of.