The Great Divide

The Xi'an Civil War, called by the Xi'an itself ' The Great Divide'  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 20044 BCE and ended 19755 BCE, -20044 to -19755 SEY after 289 years.

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 fought in endless feuds against each other. At these times, Matriarchs (woman 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 with the goal of killing the leadership of House Uai'i and Ru'a during peace talks. The entire atmosphere of the planet collapsed and killed most of its 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'.