The First Civil War is a conflict that took place prior to the creation of the Empire of Mankind but that was crucial for its future formation. This war took place between 25,490CE and 25,677 CE (5,324 ADL-5,511 ADL of the Lewisian Calendar) in the Republic of Anthropos. This nation was born out of the Republic of Elpida, that had ownership over Elpida, Mitra and Pax and its former Colonies, that had become well populated and important in their own right by the time of the conflict. Still, exploration and colonization outside of what would be known as the Imperial Core Worlds was in their initial stages with just some colonies outside the core starting to be founded by the time of the Conflict.
Two parties rose to power during these early days. The Yellow Haras Party and the Purple Thonan party. The parties had been escalating in tensions and economic power and influence for centuries, to the point it was no longer sustainable, and both parties wage open war against each other by 25,490 CE. All started with the accumulation of wealth and the Elections that took place in 25,489 CE, leading to discrepancies between the poll results and conflict of interests, that finally escalated into a declaration of war. The War was rooted among the population. Some sided with one party and others with the other party. At the same time, the military stayed aside, and most of the war vessels, personnel etc... was fund by private company support, local party resources and accumulative wealth that the parties had been gaining after centuries.
The conflict was cruel and bloody, and it was stopped by a military coup led by Horis Tarnos, leader of the Military forces. He restituted the Republic, but it was transformed, forbidding bipartidism, with a minimum of five parties in action at all times. Horis was incredibly popular and was accounted as a figured called, the Guardian, and head of the Military. His purpose was to guard the republic and its people. Later, the Guardian figure was made hereditary.
Some of this event would still prove ineffective and give rise to the Second Civil War, that led the Republic to become an Empire.