What lessons can be learned from the defeat of Odysseus and his men at Ismarus?
In the ninth book of Homer's epic The Odyssey, Odysseus and his men are trying to return home to Ithaca. They have left Ilios and landed on Ismarus, home to a people known as the Cicones. There are several lessons that may be learned from their eventual defeat.
When they land at Ismarus, they attack the inhabitants, stealing their wives and their treasure. Odysseus divides the spoils evenly among the men and tells them to leave right away. Odysseus is wise, his patron goddess being Athena, and by now his men should recognize that and listen to him. The fact that they don't shows that they are given to greed and gluttony. They want to stay at Ismarus and enjoy the pleasures found there. Odysseus is much more focused on the goal, returning home. So one lesson that can be learned is to listen to those in authority who are wiser.
Another lesson that can be learned is that giving into greed can cause you great misery. The men had received treasures. If they had been satisfied with that, they could have returned home to Ithaca victorious and wealthier. Because of their greed, many did not return home at all.
The men also acted in arrogance, so another lesson could be that pride goes before a fall. They didn't even think about the possibility there were more Cicones who would come to avenge the fallen. Surely Odysseus thought of this or he wouldn't have told them to leave swiftly. In their arrogance, they believe they have been victorious and that nothing and no one can defeat them now, so they can stay and revel in their superiority. Things end in tragedy rather than revelry, however. Below is the textual evidence for this analysis.
But come, let me tell thee also of my woeful home-coming, which Zeus laid upon me as I came from Troy. From Ilios the wind bore me and brought me to the Cicones,  to Ismarus. There I sacked the city and slew the men; and from the city we took their wives and great store of treasure, and divided them among us, that so far as lay in me no man might go defrauded of an equal share. Then verily I gave command that we should flee with swift foot, but the others in their great folly did not hearken.  But there much wine was drunk, and many sheep they slew by the shore, and sleek kine of shambling gait. Meanwhile the Cicones went and called to other Cicones who were their neighbors, at once more numerous and braver than they—men that dwelt inland and were skilled  at fighting with their foes from chariots, and, if need were, on foot. So they came in the morning, as thick as leaves or flowers spring up in their season; and then it was that an evil fate from Zeus beset us luckless men, that we might suffer woes full many.
There are several lessons that can be learned from the defeat at Ismarus. First, the defeat is the result of an unprovoked attack which may be a lesson in itself. Nothing is for free and when Odysseus attacked to gain booty and he paid for it with the deaths of many of his men. The second lesson which could be learned is that Odysseus cannot trust his men to obey him. He told them to leave quickly but in their greed they did not obey and they paid for it dearly. This would not be the last time that his men would get themselves in trouble by disregarding Odysseus' orders. A final lesson is about greed. His men wanted more and more booty and they paid for it. Again, this would not be the last time that greed gets Odysseus or his men in trouble. The defeat at Ismarus sets the scene for the repeat of these mistakes again and again. Only Odysseus begins to learn the lessons to be gained here.