Why did the Trojan war occur?

It started when the goddess of discord ERIS threw a golden Apple marked TO THE FAIREST amidst the banqueting hall of Mount Olympus. The goddesses clamored to get the Apple, that Zeus finally had to intervene. The choice was narrowed down to three: Hera, Athena and Aphrodite. He then told them that the best judge would be the Trojan Prince Paris. Paris' judgment was not according to their physical attributes but on whose gift he would favor most of all. He finally gave the Apple to Aphrodite who promised him the most beautiful woman then alive, Helen of Sparta. With her help they abducted Helen and brought her to Troy.

Helen's husband Menelaus, together with Agamemnon, summoned the kings of the different isles of Greece including Odysseus and the mighty Achilles.