In 2370 the Federation and the Cardassian Union signed a treaty that set a new border and created the Demilitarized Zone, leaving colonists on the wrong side.
After years of war and an uneasy truce, the Federation and the Cardassian Union agreed in 2370 on a treaty that fixed a new border. Several Federation colonies ended up in Cardassian territory, and some Cardassian settlements fell on the Federation side. A Demilitarized Zone was created between the two powers.
Many colonists were ordered to relocate. On Dorvan V, Picard was ordered to remove a community of settlers of Native American descent. They chose instead to give up their Federation citizenship and live under Cardassian rule. Wesley Crusher left Starfleet Academy during the dispute to stay with them.
Other colonists refused to move and took up arms, especially after Cardassian forces began attacking them. These settlers formed the Maquis, who fought both Cardassia and the Federation. The treaty thus brought peace between governments while creating a new conflict along the border that drew in many Starfleet officers.