Annexation was the forceful incorporation of a region of space or land into another political entity.
In the late 21st century, the Vulcans annexed a planet colonized by the Andorians and named it Paan Mokar. (ENT: "Cease Fire")
The Arin'Sen colony of Raatooras was annexed by the Klingon Empire in the mid-22nd century and became a protectorate. (ENT: "Judgment")
The Romulan Star Empire annexed a planet in 2152. (ENT: "Minefield")
In the mirror universe, the Tholians frequently annexed star systems outside their core territory. (ENT: "In a Mirror, Darkly") In the prime universe, the Tholians claimed an area of space as their territorial annex where the USS Defiant disappeared in 2268. (TOS: "The Tholian Web")
In 2328, the Cardassians annexed Bajor, beginning the Occupation of Bajor. (TNG: "Ensign Ro")
In one possible timeline, during the first half of the 24th century, the Zahl defeated the Krenim and took back the planets previously annexed by the Krenim. (VOY: "Year of Hell")
During the Federation-Cardassian War, the Cardassians tried to annex the Minos Korva system. (TNG: "Chain Of Command, Part II")
In 2372, during the Klingon-Cardassian War, the Klingons sought opportunities to distract the Federation so the Empire could annex more Cardassian space. (DS9: "Rules of Engagement")
In early 2373, Gowron demanded the Federation allow the Klingons to annex Archanis IV and other worlds seized in order to end the Federation-Klingon War. (DS9: "Apocalypse Rising")
According to The Voyager Encounter, Captain Kathryn Janeway allied the warship Voyager with the Vaskans in 2374 so the Vaskans could annex Kyrian land. (VOY: "Living Witness")