The Novels say the Mars has a seat on the Federation Council.
Each Federation Councillor is a representative of a distinct empire that may have thousands of worlds to it's credit, or might have invented Warp only a year ago.
The Maquis worlds, Federation Colonies were ceded to the Cardassians when the border moved.
The Maquis chose to become Cardassian citizens rather than be relocated to new Federation worlds, because home is home.
The Cardassian Government promised to treat it's new human citizens like it's regular Cardassian Citizens, and Federation believed them.
Xenophobic hillbillies (on both sides) who don't give two shits about how the Government tells them to think and behave, didn't give two shits about how the government told them to think and behave.
The Maquis (armed by sympathetic corners of Starfleet) were killing Cardassian farmers living on former Federation worlds, and the new Cardassian farmers on former Federation worlds where being armed by black sections of the Cardassian government to kill the Maquis (as well as regular nonmilitant farmers who just wanted to farm) or at least keep the Hillbilly brawl at level where neither side were outmatched, in limited "civilian" engagements.
Then the Maquis started killing Cardassians military/police who were trying to keep order and make sure that murder and war wasn't happening. If the Cardassian Government was fair, they should have been sending both sides of these altercations to penal colonies work camps, and maybe they did, but the maquis argument was that there was bias and only the former Federation Citizens were being punished for at least half the time, only defending themselves.
Because of extradition laws and a formal alliance, Starfleet was honour/legally bound to treat Cardassian criminals as if they were federation Criminals in need of being apprehended.
The Maquis, now like a (small) modern military power (Hillbilly plus) was then zig zagging across the border (when they were not relieving themselves in the Demilitarized Zone) to evade simultaneously and sequentially Starfleet and the Cardassian Union depending on who was chasing them at any given moment.
This was always almost more about murder and theft, since the Maquis had no infrastructure and could not surviving without raiding the crap out of Cardassian interests, more so than politics.
The Hatfields and McCoys in space.
The Maquis denial (rejection?) of the 2267 peace treaty is nothing like Brexit, unless 5 million British refuges armed like a soccer riot, show up in Normandy next week, and decide "fuck it, lets just loot and pillage until the French wimp out and give us Normandy".