All Hell breaks loose.
This was a great episode, which aired primarily on UPN stations in May 1997 (and yes WWOR in New York), as the finale of the 5th season. And to the morons who are stupid, yes without the Dominion, the Cardassians were far more advanced than the major 3 anyways, as the Federation fought a brutal war with them to a stalemate, and Picard in "Journey's End" is frought with fear at the thought of fighting them again, far more than he is with the Romulans.
As for why an advanced enemy needs to team up with the Dominion it's that simple. This was not some thing that happened overnight. It started in the very beginning. The Cardassians have discovered the wormhole in "Emisary" and Dukat is interested in orbs (he interrogates Sisko about it in the pilot). Ever since the beginning of this show, the Cardassians have taken a special interest in the wormhole, seeing possibilities of endless immortality and power, eerily reminscent of the Spanish and their quest for the fountain of youth. Dukat egged this on for decades, and joining the Dominion was their chance to galaxy domination. Most of the episode deals, however, with the preparation for war, with the various conflicts with the many different species that occupy Terek Nor (as Deep Space Nine). We see Quark getting ready. What follows after is the Dominion reaching Deep Space Nine. If the Cardassian-Federation War was all Hell breaking loose, this one is that phrase 10X more. It's Armegeddon in the worst ways. The Dominion, the Cardassians, the Jem'Hadar, the Breen all fighting the Federation. It's a fairly brutal episode. And one of the best. UPN/Warner Bros./WB flagship. And this show was one of the pioneers in using reused footage, a UPN invention.
