Legend has it that the chain of events leading up to the Trojan War started at a royal wedding. Peleus, king of the Myrmidons, a race of people created from ants, was marrying a sea nymph named Thetis. Many gods attended the wedding, but Eris, daughter of Zeus, king of the gods, wasn't invited. She was the goddess of discord and they feared she would only stir up trouble.
Out of anger at being excluded, Eris decided to disrupt the wedding banquet. She threw a golden apple marked "for the fairest" among the guests. The goddesses Hera, Athena and Aphrodite were there, and each thought that she was the fairest of them all. They bickered for a while about who deserved the apple, then asked Zeus to decide the matter. Zeus wanted no part of this and got Paris, son of King Priam of Troy in Anatolia to be the judge.
Peleus and Thetis didn't invite Eris to their wedding, so she sent the golden apple that she knew would cause a fight
Eris, personification of discord, threw a golden apple with the words "To the fairest," at the wedding of Peleus and Thetis.
It was at the wedding of Thetis and Peleus to which the golden apple "for the fairest" was found which Aphrodite, Hera, and Athena all fought over.
The goddess Eris possessed the Golden Apple of Discord. She caused turmoil with it at the wedding of Peleus and Thetis. Hera, Athena, and Aphrodite fought for the apple inscribed "to the fairest" which led to the Trojan War.
Eris, the goddess of discord, was responsible for that apple. She threw it as revenge for not being invited to the wedding of Peleus and Thetis.
Eris the spirit/goddess of strife and discord. She was upset because she was not invited to the wedding.
Hera, Athena, and Aphrodite were the 3 goddesses competing for Paris' golden apple and to see who is the fairest of the 3 goddesses. Paris chose Aphrodite and gave her the golden apple.
Eris, goddess of discord, threw a golden apple inscribed "To The Fairest" to the goddesses at the wedding of Peleus and Thetis because she was angry at not having been invited to the celebration. 3 goddesses laid claim to the apple, and their rivalry brought about the events which led to the Trojan war.
During the wedding feast of King Peleus and Thetis, Eris, the goddess of discord, caused chaos by throwing a golden apple inscribed with "To the fairest" into the gathering. This act sparked a dispute among the goddesses Hera, Athena, and Aphrodite, each claiming the apple for herself. Their rivalry ultimately led to the judgment of Paris, which set off a chain of events culminating in the Trojan War. Eris's intervention highlighted her role as a disruptor and instigator of conflict among the divine.
The apple of discord is a reference to the Golden Apple of Discord which, according to Greek mythology, the goddess Eris said would go "to the fairest" at the wedding of Peleus and Thetis, this started a dispute between Hera, Athena and Aphrodite that eventually led to the Trojan War. An apple is not mentioned in the Adam and Eve story, so it is unlikely that they are related.
The exact answer is Zeus. It was Zeus who wanted to couple with Thetis, but was warned by Prometheus that she would bear a son that would be greater than his father. Fearing for his throne, Zeus gave Thetis up to Peleus, one of the Argonauts. It was at the wedding of Thetis and Peleus that the Eris threw the golden apple. Zeus also selected the judge of who would win the Golden Apple: Paris of Troy. In doing so, he set in motion the wheels that allowed Paris to take Helen as his bride (for giving the apple to Aphrodite, she promised him the hand of the most beautiful woman in the world in marriage). So despite thinking it was Paris' fault, or Aphrodite's fault, or even Eris' fault, in the end, it was really Zeus' fault.
An apple of discord is a reference to the Golden Apple of Discord which, according to Greek mythology, the goddess Eris said that she would give "to the fairest" at the wedding of Peleus and Thetis, sparking a vanity-fueled dispute between Hera, Athena, and Aphrodite that eventually led to the Trojan War. Thus, "apple of discord" became a euphemism for the core, kernel, or crux of an argument, or for a small matter that could lead to a bigger dispute.