This episode effectively begins the war with the Dominion. If you want to read about the buildup, you can check out the episode recap or, better yet, Tivo the episode when it's on Spike. The flash point has to do with the Federation mining of the Wormhole. Without access to the Gamma Quadrant, the Dominion will be restricted to its ships and resources in the Alpha Quadrant. Sisko does this, knowing that the Dominion will fight.
This is truly one of the show's best episodes, largely because of all the plot threads and character details it opens up for next season. Rom and Leeta are married. Worf goes to serve with the Klingons, separating him from his new fiancee, Jadzia. Rom begins his life as a spy. Jake stays on DS9 (now officially Terok Nor) to write about the occupation. Garak comes with Sisko and the others. Sisko makes a rousing and (no pun intended) defiant speech, where he promises that he will be back, and to seal the promise, leaves his baseball on his desk. Dukat gets to strut, Weyoun gets to fret, and Quark gets to suck up to both of them, in one of the episode's lighter moments. It all adds up to one of the most exciting and densely plotted hours of Deep Space Nine yet, and the best Star Trek cliffhanger since "The Best of Both Worlds". Simply put, one of DS9's finest hours.