The Cardassian occupation of Bajor, and the Bajoran resistance was a contentious political issue for Starfleet and the United Federation of Planets leading up to the events of Star Trek: Deep Space Nine. The central premise of DS9 revolved around the political situation following Cardassia's withdrawal from Bajor. Commander Benjamin Sisko (Avery Brooks) was sent to Deep Space Nine, a former Cardassian space station, to prepare Bajor for entry into the Federation.
Ultimately, Bajor never joined the Federation in DS9, and it's easy to see why they didn't when looking back over the blind eye turned to Bajor's historical treatment by the Cardassian Union. The controversy surrounding Starfleet and the Federation's appeasement of the Cardassian Union provided Star Trek with some of its most fascinating characters and storylines. For example, Star Trek: The Next Generation introduced the conflict via the disgraced Bajoran officer Ro Laren (Michelle Forbes), whose childhood was defined by the brutality of the occupying Cardassian forces, including being forced to watch her father being tortured. Here's the history of the Cardassian Occupation of Bajor, and how it set up DS9.
