Patroclus was the son of Menoetius in Greek mythology, and best friend of the hero Achilles. His father was one of the Argonauts, while many different women have been mentioned as Patroclus' mother; Polymele, Sthenele, and Periopis.
When he was young, Patroclus had an argument with one of his friends, Clysonymus, while playing dice; in the heated argument that followed, Patroclus accidentally killed him. To evade the wrath of Clysonymus' family, Menoetius took Patroclus and they both fled to Phthia, where King Peleus reigned. There, Patroclus met Achilles with whom they formed a deep friendship, and Peleus sent both of them to be tutored by the Centaur Chiron.
Both Patroclus and Achilles were prospective suitors of Helen when she was about to get married; thus, they both took the Oath of Tyndareus, according to which all suitors of Helen would protect her and her future husband from any threat or danger. When Helen eloped with Paris, prince of Troy, Menelaus, who had married Helen, invoked the Oath of Tyndareus, thus getting the support of all previous suitors, including Patroclus and Achilles. This was the start of the Trojan War.
During the Trojan War, Patroclus was a valiant soldier. When the Trojans had taken the advantage and were threatening the Greek ships, Patroclus convinced Achilles to let him become the leader of the army and repel the enemy. So, Patroclus wore Achilles' army and led the soldiers to battle successfully. However, he did not listen to Achilles' advice to fall back once the Trojan army had been repelled; instead, he pursued them, killing many heroes that fought on the side of Troy. At that point, though, the god Apollo stunned Patroclus and was eventually killed by Hector, son of the Trojan king, Priam, and brother of Paris.
Once Patroclus' body had been retrieved by the Greeks, Achilles attacked the Trojans in fury and managed to kill Hector. However, instead of letting the Trojans bury their commander respectfully, he desecrated the body by dragging it with his chariot.