A coup d'état or simply coup is the sudden and unlawful overthrow of a government by a small group within the state establishment. The phrase means "a strike against the state" in French.
In 2372, Vice Admiral James Leyton attempted a military coup of the Federation due to his view that the civilian government was not doing enough to prepare for a possible conflict with the Dominion. He was stopped before he could seize control by Captain Benjamin Sisko and the imminent arrival of the USS Defiant with evidence of his plot. (DS9: "Homefront", "Paradise Lost")
- In the Pendragon universe, Leyton's coup was not averted and he took control of the Federation leading to the Federation Civil War. (Star Trek: Pendragon: "True North")
In 2379, Shinzon, Senator Tal'Aura t'Kaveth, and a cadre of Romulan Star Navy officers used a miniature thalaron generator to assassinate Praetor Hiren and most of the Romulan Senate, taking over the government of the Romulan Star Empire. The aftereffects of their coup led to a series of civil wars that lasted into 2410. (Star Trek Nemesis; Bait and Switch: Emael Mosekhesailho; The Path to 2409; Star Trek Online)