10. The Romulans Join The Fleet
In Star Trek Deep Space Nine's fifth season, the Dominion War was fast approaching, brought forward by Gul Dukat's treachery against the Alpha Quadrant. The alliance between the Cardassian Union and the Dominion spells dark times, as the Klingons are quickly beaten back out of Cardassian territory.
Deep Space Nine, the Federation Task Force and the Klingon Fleet begin to dig their entrenchments around the station, preparing for battle in the episode In Purgatory's Shadow. It was one of the finest episodes for building tension in the run of the show. What the audience expected was, possibly, the beginnings of a stand off between the Dominion and the Federation.
What they did not expect was for an entire Romulan Fleet to de-cloak beside the station, asking to join the, against the Dominion.
It was a cheer and punch the air moment, as the Romulans, longtime adversaries of the Federation (and the Klingons in particular) had been one of the major powers in the Alpha and Beta Quadrants. For a moment, it looks as though victory against the Dominion was all but a certainty.