Madara
He couldn't let his brother's death go. Additionally, he broke peace plans once they had finally been established...apparently planning the moon's eye plan.
After this he was the one who trained and planted ideas into Obito's mind...eventually dying to wait and entrust HIS plans to Obito. Eventually he was re-summoned, and the war likely only continues because he was there to prevent the Gokage from arriving.
Without Madara none of the events in this story would be happening.
Unless....you wish to go all the way to the SO6P's decision of the younger son...then it's his fault for all the hostility between Uchiha and Senju.