Paris runs away with Helen to Troy. The Greeks thought Helen was kidnapped by the Trojans so they pursuit them back to Troy starting as whats now as the "Trojan War"
This is the first battle between Achilles and Hector, while the Greeks and Trojans are fighting.
Achilles cousin, Patroclus did not like Achilles standing by, so he steals his armor and goes off to fight Hector, Hector ends up killing Patroclus.
Achilles being very angry that his cousin was murdered, he demanded a one on one brawl with Hector, Hector accepts and faces defeat is drug behind Achilles's chariot in humilty.
The Greeks come up with the grand scheme of gifting the Trojans a hollowed out horse full of Greeks finest warriors as a gift from the gods, they waited till the end of the festivities and got out of the horse and let the entire Greek army into the city of Troy.
The Greeks killed anyone in sight, and caused mass destruction that night, Achilles had saved a Trojan woman that he had fallen in love with, and died to Paris while thinking he was harming the woman, and not saving her.