The 24th century, defined in the Earth calendar as being from 2300 until 2399, was a period of great conflict between the various races and organizations of the galaxy. The Federation made contact with several powerful new races, such as the Borg, the Dominion and the Cardassian Union, all of whom attacked the Federation on several occasions. The Romulan Star Empire entered a fifty-year isolation, from which they emerged far stronger than before. The Borg attempted to assimilate Earth twice, in 2366 and 2373. 2373 also saw the beginning of the Dominion War, which ended in 2375, with the major powers of the Alpha and Beta Quadrants joining together to defeat the Dominion.