Who was really responsible for the Trojan war?

Eris goddess of strife and discord, because she is the one to start the golden Apple argument, and have the Athena, Hera and Aphrodite start arguing for who should deserve the Apple. This is the root cause of the Trojan war, which was caused by Eris.

OR

Paris because he took Helen from Menelaus so Menelaus got mad and started the war to get Helen and take the Trojans city, Troy