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In the Star Trek universe, the region known simply as the "Demilitarized Zone" is a "buffer" zone between the United Federation of Planets and the Cardassian Empire, in which no warships or military bases are allowed. It was established by the Federation-Cardassian Treaty of 2370, following the war between those powers. The treaty also redrew the boundaries of both, putting some former Federation colonies under Cardassian control, and vice versa.

This transfer of power was not welcomed by many living on the colonies in question, especially Federation colonists who were now forced to either submit to an oppressive Cardassian regime or leave their homes. Many rallied to the banner of a group of terrorists (or freedom fighters, depending on your viewpoint) known as the Maquis, which strove to drive the Cardassians out of the Demilitarized Zone. The Cardassians formed a similar group (initially funded in secret by the Central Command), and so the two groups set about starting their own little war.

This situation waxed and waned for several years, with the Maquis eventually gaining the upper hand (with both Cardassia and Starfleet unable or unwilling to dedicate the resources necessary to stop them). However, in 2373 Gul Dukat wrested power from Cardassia's new civilian government, and proclaimed that the Cardassian Union was now a member of the Dominion. Shortly before the Dominion War broke out, the Jem'Hadar swept through the former Demilitarized Zone and utterly destroyed the Maquis; virtually none of them survived.

It is not known what the current status of the Demilitarized Zone is; the matter was not resolved at the end of Deep Space Nine series, and neither Star Trek: Voyager nor the Star Trek movies following DS9 have dealt with the Cardassians in anything more than a passing reference. Presumably the new Cardassian government, run by members of Damar's resistance movement, is quite aware of the aid the Federation gave them in retaking their worlds (even if its actions were not truly altruistic), and one would expect that tensions between the two powers would have lessened. Still, only time (and a replacement to Brannon Braga and Rick Berman) will tell.