How It Came To Be
The Sinestro Corps War
It began with Sinestro, who harnessed the yellow light of fear to power his own corps. His Sinestro Corps waged a bloody war against the Green Lantern Corps, killing hundreds of Green Lanterns. These actions provoked the Guardians of the Universe to enact ten new laws to prevent the War of Light from happening, and they began with authorizing the use of lethal force. The Sinestro Corps lost the war which culminated in an enormous battle on the planet Earth, and Sinestro was captured along with many of members of his Corps. The rest of his Corps scattered throughout the universe. But the damage had already been done. Sinestro's war was the spark needed to ignite the War of Light.
Sins of the Star Sapphires
Inspired by the efficacy of the Guardians' Green Lantern Corps, the Zamarons reforged their Star Sapphire alien gems into rings and constructed a Central Power Battery on Zamaron fueled with the violet light of love. The Zamarons philosophically disagreed with the Guardians' methods of imprisonment and punishment. Star Sapphires began soaring through the universe, hunting down members of the Sinestro Corps to subject them to a forced conversion process that would turn them towards the ways of love and the ways of the Star Sapphire. The Guardians attempted to negotiate with the Star Sapphires' leader, Queen Aga'po, but she refused their demands to pull back her Corps. The Star Sapphires would continue their advance on the universe, as well as their campaign against the Sinestro Corps.
Rage of the Red Lanterns
Yet another corps lit its light in the universe. Atrocitus discovered how to burn the red light of hate and founded his Red Lantern Corps on Ysmault, the prison planet he had been sentenced to by the Guardians. He populated his Corps with beings who had suffered terrible wrongs at the hands of the Sinestro Corps and had an overwhelming desire for vengeance, as he personally did for Sinestro. The Red Lanterns made their debut by tearing through Sinestro Corps and Green Lantern alike to abduct Sinestro from his imprisoned transport to Korugar. Before Sinestro could be tortured to death on Ysmault, Hal Jordan of the Green Lanterns, a unit from the Sinestro Corps and two members of the new Blue Lantern Corps descended upon the planet to rescue him. Atrocitus plotted revenge against the Blue Lanterns, casting blood magic rituals to locate their homeworld, and the Guardians also plotted retaliation against him, making preparations to dispatch their Alpha Lanterns to Ysmault.
Raid on the Vega System
The Controllers sought out the orange light of avarice on the planet Okaara of the Vega System and were unprepared to discovered it had already been harnessed into an Orange Lantern Corps by a being named Larfleeze. This was the final straw for him, as already both the Green Lantern Corps and the Sinestro Corps were trespassing on his territory. The Vega System was promised to him by the Guardians if he kept his Orange Lanterns buried there, and he threatened that he would no longer do so if they did not meet his demands. The Guardians responded by calling off their pact with Larfleeze completely and launched an all out assault on the Vega System to take what was his. On Okaara, Larfleeze retaliated by releasing his own Orange Lanterns against the Guardians and their Green Lanterns. The two forces clashed and wound down to a draw, with the Guardians negotiating an end of hostilities with Larfleeze. He wanted only one thing to end the conflict. He wanted to know where one could get a blue ring like the one Hal Jordan had worn.
The War of Light Explodes
After all of these individual skirmishes, the War of Light began raging across the universe. On the Blue Lantern Corps homeworld of Odym, Ganthet and Sayd were inducting their fourth member when Larfleeze descended upon them with his entire Orange Lantern Corps. Elsewhere, the Yellow and Violet clashed as Sinestro led his forces to Zamaron to reclaim his captured people from the Star Sapphires. More war wages on Ysmault, where the Lost Lanterns and Alpha Lanterns of the Green Lantern Corps came to lay siege upon Atrocitus and his Red Lantern Corps. Only the Indigo Tribe remained uninvolved thus far, but the Blackest Night had come.