The Demilitarized Zone (sometimes referred to as the DMZ) was a buffer zone in the Alpha Quadrant. The zone was established between the United Federation of Planets and the Cardassian Union, by the terms of the Federation-Cardassian Treaty of 2370. According to the treaty, neither side was permitted to deploy armed starships, or establish military bases in the Zone. In addition, the border between the two powers was redrawn, such as that ownership of several colonies was exchanged both ways – some Cardassian colonies were transferred to Federation control, and vice-versa. The colonists on the affected planets were given the option of resettlement, though most refused. Dorvan V was one of the Federation colonies that was turned over to the Cardassians. (DS9: "Whispers", TNG: "Journey's End")

However, many civilian colonists on both sides of the border objected to the transfer of control and secretly armed themselves, violating the peace terms that established the DMZ. The Cardassian Central Command began secretly shipping weapons to their colonists in the DMZ so as to oust the Federation colonists, using neutral species such as the Xepolites as intermediaries. In response, the Federation colonists banded together and called themselves the Maquis, dedicated to freeing themselves from Cardassian control. Although the Maquis gained no official support from the Federation, many in the Federation (including Starfleet) were sympathetic to their cause and covertly provided aid. (DS9: "The Maquis, Part I", "The Maquis, Part II"; TNG: "Preemptive Strike")

However, by late 2370, to help reduce illegal shipments, the Federation had established a series of checkpoints and space buoys along their side of the border. Any ship that crossed anywhere other than a checkpoint would be investigated by Starfleet. (TNG: "Preemptive Strike")

For the next three years, the DMZ was a hotbed of small-scale warfare as the Cardassian and Federation colonists conducted terrorist operations against each other. Eventually the Cardassian military became directly involved in the Maquis conflict on their side of the DMZ. However, in 2372 the Cardassians were forced to divert the majority of their attention to defending against the Klingon invasion in the Klingon-Cardassian War, leaving the Maquis practically unchecked in the DMZ. (DS9: "For the Cause") By 2373, the Maquis had been so successful that their colonies were planning to declare themselves an independent state. (DS9: "Blaze of Glory")

The Demilitarized Zone was quickly overrun by the Jem'Hadar in 2373, shortly after the Cardassian Union joined the Dominion. As the Dominion moved towards open warfare with the Federation, the Demilitarized Zone ceased to exist and became Cardassian territory. (DS9: "By Inferno's Light")

Planets in or near the DMZ

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