Maquis
The Maquis are a terrorist group of ex-Federation citizens living in territory that was ceded to the Cardassian Union following the Cardassian War.
History
At the end of its war with the Cardassian Union, the Federation agreed to a new border, with some territory that belonged to the Federation before the war transferred to the Cardassians. A few Federation citizens living in colonies in the ceded territory refused to leave, and the Federation allowed them to stay, notifying them that by doing so they surrendered their Federation citizenship and would receive no support in the event of Cardassian oppression.
Cardassian oppression soon followed, and true to its word, the Federation refused to assist its former citizens. The isolated colonists managed to acquire their own weapons and space ships, however, with which they fought a guerilla campaign against the Cardassians. Surprisingly, the Federation responded to Cardassian demands that they help suppress the rebellion of their former citizens, leaving the Maquis fighting a two-front guerilla war against both the Cardassians and the Federation.
The Maquis and all of the former Federation colonists were eventually wiped out by the Jem'Hadar after the Cardassian Union, reeling from attacks by the Klingon Empire, decided to ally itself with the Dominion.