Call to Arms, DS9 5
And so, after many false starts, the Dominion War has begun!
And, like, holy effing shit!
Worf leaves with Martok, Leeta is sent to Bajor, Sisko and the rest of Starfleet run off with the fleet and Quark, Garak, Odo and Kira are left on a deserted (and half-destroyed) Terok Nor as Dukat and Weyoun stride in with their pyrrhic victory.
Best episode ever.