That's correct. In the 2386 and 2387 history updates for the game they mentioned that the Cardassians signed a treaty with the Federation that they would disband their military forces and Starfleet would then be responsible for defending the Union from invasion and attack. Their entire military isn't gone, they retain a small defence force.
I've c&p the relevant sections:
2386: In addition to working to curb the rising hostilities between the Klingons and the Gorn, Federation diplomats are hard at work crafting a new pact between the Federation and the Cardassians. On Stardate 63976.74, the Cardassians sign a new treaty, in which they agree not to field a military or wage war. In exchange, the Federation agrees to provide aid to rebuild Cardassian cities and protection in the event of an invasion of Cardassian space. In response to the landmark agreement, the Bajoran coalition drops its request to prosecute Cardassians for war crimes. Many among the Bajorans and Cardassians hope that someday relations between them will be normalized.
2387: Starfleet sends a fleet to Cardassia Prime to assist the Cardassians with the dismantling of their military, a process which is expected to take several years. The Cardassians plan to use a much smaller self defense force to patrol their space, and coordinate with Starfleet for issues outside of their borders.
Noooo! Why does it have to be the Cardassians! Let them take someone useless like the Klingons or the Borg. There destroying a great race that was created with alot of character depth.
They won't even be a 3rd rate power in Star Trek Online. What am I suppose to play a Romulan???