Chyrellka

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Chyrellka, also known as Chrellkan IV, was the fourth planet in the Chyrellkan system of the Chyrellkan Sector. Despite the fact that the Chyrellkan people were human, they only achieved contact with the Federation in 2259. Shortly afterwards, Klingon agents, including rogue Starfleet officers, infiltrated Chyrellkan society and that of their colony world, Vancadia, attempting to start a war. However, in 2269 James Kirk and crew exposed the Klingon plot and the war was averted.

The Chyrellkans were technologically inferior to the Federation, but were a peaceful people. Chyrellka had not experienced a war since 2069. (TOS novel: Renegade)

The book never explains how Chyrellka came to be inhabited by Humans.