The war of light began with the Sinestro Corps war. I've already told you all about sinestro in previous posts, so if you don't know who that is, there's plenty of posts to read about him.
In the beginning, Sinestro used the yellow light of fear in order to give energy to his Sinestro corps, who began a war against the Green Lantern corps. In this war made up of many battles, hundreds of Green Lanterns were killed. These deads made the Guardians of the Universe take part and create new laws to prevent the War of Light from happening, and they allowed lethal force to enforce these new laws. The Sinestro corps ended up losing this war in an epic fight between the two teams on earth and eventually sinestro was made prisoner alogside many other team members, while several others ran away and hid away in the universe. Although the war of light war prevented at the beginning and the defeat of the sinestro corps helped, the spark was already ignited and the war of light would be inevitable sooner or later.
