Why does Agamemnon urge the Achaeans to return to Greece?

You can find the answer to this in book 9, when the Achaeans have been routed by the Trojans and beaten back to their ships (which the Trojans plan to set fire to, trapping the Achaeans on the sea-strand to slaughter them there). Achilles has been sitting out from fighting for the past 8 books, and the tide of war has gone from a stalemate to a state of absolute devastation for the Achaeans. Agamemnon at this point determines that this is a loss from which they cannot recover, and he suggests that the armies should just give up and flee while they still have the chance. Diomedes stands up and says he’s going to stay until the end, no matter what, and this rallies the troops a bit. Agamemnon then decides that maybe he could try to make up with Achilles and get him back on his side. (This does not work, after which point Patroclus, who feels terrible for the soldiers, dons Achilles’s armor and goes out pretending to be him to raise the troops’ morale. Patroclus is then killed.)