The apple of discord belonged to Eris, the Greek goddess of discord and strife. When the wedding of Peleus and Thetis took place, all of the gods and goddesses were invited. Because of her unpleasant nature, Eris was not invited. She appeared at the wedding anyway.
Enraged at not being invited, Eris threw a golden apple onto the wedding table between Hera, Athene and Aphrodite. Each of the goddesses laid claim to the apple, which bore the inscription "For the fairest." Zeus did not want the responsibility of judging between the three most powerful goddesses. He declared that the Trojan prince Paris would act as the judge.
Zeus sent Hermes to request help from Paris, who agreed. Each of the goddesses attempted to bribe him. Hera offered power and wealth; Athena offered victory in battles, glory and wisdom; and Aphrodite offered the most beautiful woman in the world. Paris gave the apple of discord to Aphrodite, who promised him Helen, the wife of the Spartan Menelaus.
Hera and Athena became the enemies of Paris and all of Troy when he gave the apple of discord to Aphrodite. When Paris stole Helen from her husband, the Greeks went to war against the city of Troy, with Hera and Athene fanning the flames. From the moment Eris threw the apple of discord between the three goddesses, war was inevitable.
The modern definition of the apple of discord is "a cause or subject of strife or dissension," according to the Oxford English Dictionary. In literary analysis, it is used to describe an element or person that creates conflict within a narrative. People also employ the phrase to describe all types of conflicts, from personal to global. One example of current usage is in a news report from TopNews United Kingdom: "Falkland Islands have become an apple of discord between Britain and Argentina amid speculations that large quantity of crude oil lay under the sea beds of the disputed islands" (sic).