Formed in the 19th century power was shared between the Cardassian Central Command and Obsidian Order. The civilian Detapa Council was also created to be the chief governing body between the two but in reality had virtually no power. (DS9: "The Defiant")
Due to a lack of resources the Union became a militaristic in an effort to expand and alleviate this. One of their most noted conquests was that of Bajor in 2328. In the following decades the Union plundered the world and enslaved its people before retreating from the planet in 2369. (DS9: "Emissary")
The Cardassian Union also clashed with the United Federation of Planets along their shared border throughout the mid-24th century. Several treaties between the two were signed and in 2370 the Demilitarized Zone was created as a buffer between the Federation and Cardassia. (TNG: "Chain of Command, Part I", "Chain of Command, Part II"; DS9: "The Maquis, Part I", "The Maquis, Part II")
Following the creation of the DMZ the Cardassian Union became the target of the Maquis, a group of ex-Federation citizens, which clashed with the Union over control of the DMZ. (DS9: "The Maquis, Part I", "The Maquis, Part II"
In 2372, the Cardassian Dissident Movement succeeded in its objective and overthrew Central Command after the collapse of the Obsidian Order, putting control of the Union into the hands of the civilian Depata Council. This led to a period of unrest that saw the Union close its borders. The Klingon Empire, influenced by a Changeling infiltrator posing as General Martok, saw this as a sign of Dominion infiltration of the new Cardassian government and launched an invasion of the Cardassian Union, nearly toppling it. However, when faced with the prospect of igniting a war against the Federation as well the Klingons halted their invasion, though they refused to give up the territories they had already conquered. (DS9: "The Way of the Warrior")
Over the next year the Union was besieged by Klingon raids and a more aggressive Maquis movement. Gul Dukat secretly entered into negotiations with the Dominion and in mid-2373, the Dominion entered the Alpha Quadrant and backed a coup that saw Dukat installed as the new ruler of the Union, all in exchange for the Cardassian Union becoming a member of the Dominion. (DS9: "By Inferno's Light")
The Dominion quickly resecured the Union's borders, driving the Klingons out and wiping out the Maquis in short order. The Dominion then set about reinforcing their position with weekly convoys of ships and material from the Gamma Quadrant. (DS9: "By Inferno's Light", "Blaze of Glory", "Call to Arms"
In an alternate universe, the Cardassian Union as part of the Dominion defeated the Federation and Klingon Empire in the what would become known as the "Unification War". (TFS: Spectrum: "The Final Prophecy")
Governing the Union fell to the Detapa Council, Cardassian Central Command and the Obsidian Order. However, until 2372, the Depata Council was next to powerless in reality with Central Command and the Obsidian Order controlling the Union. The collapse of the Order following their disastrous Omaricon Nebula campaign and the rise of the Cardassian Dissident Movement saw the Depata Council take power.
Following joining the Dominion the Union was ruled by a series of installed leaders from Central Command. Gul Dukat was the first, he was replaced by Legate Damar following the Dominion's failure to gain reinforcements from the Gamma Quadrant.