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In the Star Trek fictional universe, the Terran Empire is a fascist imperialist interstellar political power, ruled by an Emperor from Earth. In stark contrast to the United Federation of Planets, the Terran Empire is xenophobic, and conquers alien races with military force. While Federation starships use "U.S.S." as a prefix, Terran Empire starships use "I.S.S.".
The parallel universe that has the Terran Empire diverged from the "prime" universe during the Vulcan first contact mission of 2063. In the "prime" universe, Dr. Zefram Cochrane offer terms of peace to the arriving Vulcans. However, in what has been nicknamed the "mirror" universe, Dr. Cochrane instead shoots one of the Vulcans with a plasma shotgun, while those around him overpower the remaining Vulcans and seize control of their ship. They rapidly reverse engineer the technology on board, and eventually launch an attack on Vulcan, making it the first world to be subjugated by the Terran Empire.
Due to reforms begun by Spock in the 23rd century, the Terran Empire left itself vulnerable, and was conquered by the Klingon-Cardassian alliance; while the Alliance maintained that it had conquered Earth, the Sol system was in fact still controlled by the Empire, with the Alliance considering it too costly to invade, opting to keep Terran forces bottled up inside the system. Due to assistance from the "prime" Benjamin Sisko in the 24th century, and from a campaign launched from earth by the "mirror" Jean-Luc Picard, the Terran Empire began offensives against the Alliance, staged from both the Bajoran system and the Sol system. By the 25th century, the Terran Empire had been restored to power, and had begun to launch trans-dimensional attacks against the Federation.