Zeus concedes that Menelaos won the fight with Paris (see the summary of Book 3 for details). He asks the other gods whether they should allow the two armies to resolve their differences peacefully, or, instead, stir up the fighting once again.
Hera says, "No way are we going to let there be peace, not after all the effort I've put into hurting the Trojans."
Zeus criticizes Hera, saying that her grudge against the Trojans has been blown way out of proportion. All the same, he says, "Go ahead, but whenever I've got a mind to destroy one of the cities you like, I'd best not find you standing in my way. After all, the Trojans have always honored me with excellent sacrifices."
Hera says, "Whatever. I like the cities of Argos, Sparta, and Mycenae. You can destroy them anytime you want—see if I care! But let's get down to business. We've got to end this truce. Let's make the Trojans break it, so they can take the heat."
Zeus says, "No sweat," and sends Athene down to rustle up some tussle.
Athene joins the Trojan ranks and takes on the shape of a Trojan fighter. In this disguise, she approaches Pandaros, the archer. She eggs him on to take a pot-shot at Menelaos.
She wins him over. While Pandaros is preparing his bow, Homer gives us the bow's backstory (made of the horns of a mountain-goat).
Once Pandaros is ready, he takes his shot. As the arrow speeds through the air, things are looking bad for Menelaos—until the goddess Athene invisibly swoops in and deflects it, so that it hits him in the hip.
When Agamemnon sees his brother bleeding (the blood running down his leg is famously compared to red dye on ivory), he gets worried.
Menelaos says, "It's only a flesh wound."
Even so, Agamemnon starts freaking out. He makes a confused (but not confusing, because we can grasp his state of mind) speech about how Zeus will punish the breakers of the truce (too bad we just saw him allow the truce to be broken), but also how if Menelaos dies the Achaians will sail home in defeat and the Trojans will lord it over them.
Agamemnon immediately starts calling out for Machaon, the greatest healer among the Achaian army.
Talthybios the herald goes off, finds him, and brings him back. Machaon attends to Menelaos's wound.
Meanwhile, the Trojans are starting to attack, and the Achaians have to prepare for battle in record time.
Agamemnon moves among the ranks, encouraging the courageous and brow-beating the cowardly.
He gives special praise to the warrior Idomeneus of Crete.
When he sees Aias and little Aias, he says he won't give them any commands—it would insult their enthusiasm.
Next Agamemnon comes upon Nestor, who is giving some tactical suggestions to the younger warriors.
Agamemnon praises him, and says if he were only a young man again, he would be an awesome warrior. Nestor agrees.
After that, Agamemnon comes upon Odysseus and some other guys who are chilling out at the back of the army. In fact, they're so far back that they haven't even heard the order to attack yet.
Agamemnon insults Odysseus and says, "What's the deal? You're always first in line when you're coming to one of my feasts. How come you're hanging back now?"
Odysseus tells him to back off, and not pass judgment until he sees him fighting.
Agamemnon says that he's just teasing him, no offense intended.
Next Agamemnon comes up to Diomedes and his buddy Sthenelaos. Agamemnon disses these guys severely, saying that their dads were much better warriors than they were.
Diomedes doesn't say anything in reply, but Sthenelaos mouths off at Agamemnon.
Diomedes tells him to keep his mouth shut. He explains that Agamemnon is just showing them some tough love.
Now the two armies are marching toward each other. The Achaians are marching in silence so they can hear the voices of their commanders. In contrast, the Trojan ranks are a cacophony of voices in different languages—representing all the Trojans' allies.
The god Ares is on the Trojan side, and the goddess Athene is on the side of the Achaians. Some minor divinities—personified forms of bad things like Strife, Hatred, etc.—are also in the mix.
At last, battle commences. There follow some gory descriptions of combat.
Aias and Odysseus each put in a good showing.
When the god Apollo sees the Trojans shrinking back, he calls out to them to cheer them up. He also lets him know that Achilleus isn't fighting for the Achaians.
There follow some more gory descriptions of combat.