Achilles' shield represents the strength and protection it offers him, and is also symbolic of the bigger picture outside war in the Iliad. Achilles receives motivation to return to war, and reason for new armor, following the death of his good friend Patroclus. His "terrible, wrenching cry" disturbs his mother, Thetis, and she comes quickly to see what the problem is. When she realizes her son's pain, and desire for revenge on Hector, she also recognizes that he will need new armor, for Hector currently flaunts Achilles' armor. Thetis returns one day later with fine armor crafted by the metal smith Hephaestus. Hephaestus has forged a breastplate, a helmet, and an extraordinary shield embossed with lifelike images. The attractiveness of the new magnificent armor seems to create a figure of equally magnificent strength.
With his new armor, Achilles agrees to return to battle, and this is metaphoric for a new hope for the Achaean army.
Being his first entry into the battle during the Iliad, this shows a change in character for Achilles. Having the realization that Patroclus will not return, Achilles develops less self-pity and selfishness. He decides to fight for his lost friend, and becomes less concerned in brooding about his own life. He doesn't return to war on the basis that his army needs him, but he returns nonetheless. One wonders if Patroclus' death was the doing of the gods, perhaps siding with the Achaeans and knowing the outcome it would have on Achilles.
Homer compacts ten years of war into two short days in his narrative, drawing in background information about the warriors and their lives. In two days he illustrates a detailed picture, and compresses important incidents. Achilles' shield becomes a contrast with the rest of the story...