Xenophobia is an irrational, unreasoning fear of that which is foreign or alien to oneself.
Throughout history xenophobia led to like minded individuals banding together into groups such as Terra Prime on Earth, the Alliance for Global Unity on Bajor, and the Vulcan Isolationist Movement. Such groups often committed terrorist activities and other crimes. Federation allied intelligence organizations such as Section 31, the V'Shar, and Starfleet Intelligence often kept a close eye on such groups, and would intervene when xenophobic groups committed crimes or terrorist attacks. (ENT episode: "Terra Prime", TNG episode: "Gambit", and DS9 episodes: "The Homecoming", "The Circle", "The Siege")