The Federation-Cardassian Demilitarised Zone (often just shortened to the Demilitarised Zone) was a region between the United Federation of Planets and the Cardassian Union. Created from a treaty that ended the Federation-Cardassian War the zone acted as a buffer between the two empires.
As part of the deal several worlds from both sides were ceded to the other. The Federation attempted to evacuate these worlds but not all the inhabitants were willing to leave their homes, instead accepting that they would be under Cardassian rule.
The Cardassian Central Command started to secretly arm its colonists in the DMZ and waged a campaign to drive the Federation colonists from their worlds. In response the Federation colonists formed the Maquis to combat the Cardassian colonists and the Cardassian Union in general.
The Klingon invasion of Cardassia, further weakened the Cardassians and the Maquis were able to make significant gains against their opponents. So much so that by 2373, they were considering making the Federation colonies an independent state.