The Great Divide

The Great Divide, often referred as the Xi'an Civil War, was a series of military engagements which ended with the eradication of the Xi'an homeworld's atmosphere, the death of 'millions' of Xi'an and formation of the Xi'an Empire. The war began in 20044 BCE and ended in 19770 BCE, and lasted 289 years (-20044 to -19770 SEY).

Background
The Xi'an society consists of different Yii'ua, the Houses, which can have the function of corporations or even state-like entities. In earlier times, the Xi'an had no central leadership, and the houses seemed to fight in endless feuds against each other. At these times, Matriarchs (female rulers) held the primary power over the Houses.

According to the Galactapedia, the conflict began when the last daughter of House Kl.ō attempted to defect to House Uai'i, which lead to further hostilities. Other big houses joined (Chii and Ru'a), which escalated to a system-wide war.

Members of House Kl.ō sabotaged the weather satellites of Hyoton III, intending to kill the leadership of House Uai'i and Ru'a during peace talks. However, the sabotage backfired disastrously by making the entire atmosphere of the Xi'an homeworld toxic, killing most of the planet's inhabitants.

Results
The loss of Hyoton III signified a major change in Xi'an history. House Ru'a became the first ruling house of the Xi'an Empire six years after the war. The calendar was reset to the imperial year 0 (I.0, 19747 BCE)

Survivors of the catastrophe were evacuated to Hyoton IV (Ka'ua), which became the new capital of the Empire and the Civil Service was established.

House Chii attempted to seize power again thirteen years later (19742 / 19734 BCE ?) but were defeated and exterminated.