Which battle led to the birth of the Khawarij?

The Khawarij sect emerged as a result of the Battle of Siffin (657 CE).

It was fought between the forces of Caliph Hazrat Ali and those of Muawiyah ibn Abu Sufyan. 

The battle ended in a stalemate, and the arbitration process that followed led to a group of rebels, who would later become known as the Khawarij, to break away from Hazrat Ali’s camp.