Atlantean-Raman War

"The end of the Harrappa-based Indus Valley Civilization in South Asia, including the atomic bombing on Mohenjo-Daro, was something that Pika just came for the very first time in Terran history. We thought the first explosions happened in the desert of New Mexico, and even in the two Japanese cities of Hiroshima and Nagasaki in the final days of World War II, but we were damn wrong. The Atlanteans and the Ramans had waged war on each other not only with ancient weapons, but with nuclear weapons."

--Su Ji-Hoon, A Brave New World

The Atlantean-Raman War was the decisive, but final conflict of the Kids' Club War that took place thousands of years before the Third World War. The war included the First Battle for Earth, as well as a subsequent liberation, which were fought for control of the planet Earth.

Prelude
During a period of time known as the Dawn of Darkness, a mad emperor under the name of Torvivum Zuskelgar came to believe that conquering the entire multiverse will allow its inhabitants to kneel before him. The Empire of Light and the Kids Club took note of this and driven Torvivum off the face of the Earth; however, Zuskelgar gained followers on the way and formed a new empire that eventually became the Empire of Darkness.

Years later, the Empire of Darkness waged war against the Light Empire and the Bronze Age civilizations, but the Light Empire continued to fend off the onslaught of the Dark Empire as the Light Empire fell and the Kids' Club War began because of this.

History
The Empire had managed to take over Earth and most the multiverse, and later the Alliance liberated the planet and ultimately defeated the Empire itself.

Aftermath
After a millenia of full-scale war with the Dark Empire, the Light Empire gradually declined and fell. The newly founded Grand Alliance united much of the multiverse under the rule of democracy until the Great Interdimensional War began thousands of years later...