Isolationism was the tendency of a state to withdraw behind its own borders and to minimize or cut contact with other races.

After they lost the Earth-Romulan War and the establishment of the Romulan Neutral Zone in 2161, the Romulans went into isolation for over a hundred years. (TOS: "Balance of Terror") They did so a second time for over fifty years after the Tomed Incident in 2311. (TNG: "The Neutral Zone")

Likewise, the 24 Great Houses of the Klingon Empire isolated themselves for almost a century prior to the outbreak of the Federation-Klingon War in 2256, with only few encounters between the Federation and the Klingons. (DIS: "The Vulcan Hello", "Battle at the Binary Stars")

The Ba'ul were known isolationists. (DIS: "The Sound of Thunder")

The Kaelons of Kaelon II were isolationists, almost to the point of xenophobia. (TNG: "Half a Life")

According to Gul Dukat, the Bajoran people were "a weak, contemplative race, choking on [their] isolation" before the Cardassians arrived. (DS9: "Indiscretion")

The Logic extremists, Terra Prime, and Vulcan Isolationist Movement sought isolation from "Alien" influence.

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