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The Seldonis IV Convention governed the treatment of prisoners of war among its signatories, which included the Federation and the Cardassians. A person could only be deemed a prisoner of war (and get the protections of the treaty) if they were acting on behalf of one of its signatories. (TNG: "Chain of Command, Part II")
In the early months of the Dominion War in 2374, Captain Benjamin Sisko and his crew crashed on a planet in a Cardassian dark matter nebula. There, they encountered Keevan and several Jem'Hadar troops. The Vorta later betrayed his men and gave Sisko their exact plan of attack. Keevan was willing to spend the rest of the war as a prisoner of war. (DS9: "Rocks and Shoals")
Later that year, several Jem'Hadar soldiers serving under Gelnon captured the USS Defiant. Sisko and his crew were able to regain control of the starship, with help from Dax, O'Brien and Bashir aboard the shrunken USS Rubicon. The Jem'Hadar were then transferred to a Federation prisoner of war camp. (DS9: "One Little Ship")