The Iliad can be understood on one level as an extended meditation on war. The moral message is that the courage to fight is what gives a man's life worth, but that war itself is a tragic situation.
Honor in this poem is defined as fighting bravely for one's homeland and fellow soldiers. Hector, for example, looks down on Paris for not wanting to fight. Hector's wife, Andromache, begs him not to go to battle, and he is deeply grieved at having to leave her and his young son. However, as he explains to her, he would die of shame if he did not fight, even though he has a strong, foreboding sense that he will be killed, Troy will be defeated, and his family will be left to the violent retribution of the Greeks. Nevertheless, he can't bring himself to stay out of the fight. Likewise, when offered a long and peaceful life or a short life of honor and fame as a fighter, Achilles chooses the latter.
While fighting honorably is what defines manhood in the context of the epic, Homer does not flinch at showing the cost of war. Men like Achilles grieve as they lose close friends, and Hector is forces to leave his wife and son. The Trojan War drags on seemingly without end, and the body count rises, causing evermore grief.
Perhaps the most poignant expression of the conflicted moral message that war is honorable but tragic is revealed by Achilles's shield. On it are depicted peaceful scenes that represent everything the men are fighting to protect and preserve: cities, fields of grain, bountiful vineyards, and worship rituals that involve dancing. That peaceful life is enabled by the willingness of the men to fight. At the same time, the fighting keeps them from that peaceful life, and its tragic losses rip them apart.