Achilles refused to help the Greeks assault Troy because he was mad at Agamemnon. The gods sympathized with him, so they enabled Hector and the Trojans to push the Greeks back all the way to their ships. This troubled Patroclus, so he begged Achilles to lend him his armor and let him join the Greeks on the battlefield. Achilles consented, but he told Patroclus to limit his efforts to defending the Greek ships; he did not want his friend to join the Greeks as they continued to push toward Troy. One reason for this is that Achilles did not want the Greeks to be defeated, but he also did not want them to prove victorious without his help.
When Patroclus went out on the battlefield, everyone thought he was Achilles since he was wearing Achilles' armor. This emboldened the Greeks and struck fear into the hearts of the Trojans. The Greeks successfully defended the ships, and they began to push the Trojans back toward Troy. Unfortunately, Patroclus did not listen to Achilles; he remained for the assault, and Hector eventually slew him.