Severed Dreams is a Babylon 5 episode from Season 3. In it, Babylon 5 is finally forced to declare independence from the Earth Alliance, due to President Clark's unconstitutional order to bomb civilian targets on Mars.
EDIT: It's also the location of possibly my favorite quote from the whole series.
To set this up, Babylon 5 and two renegade Earthforce ships have just fended off an attack by two Earthforce Omega Destroyers and two smaller ships, plus several wings of fighters, and now a new "loyal" battlegroup has just come in. B5 and her commander, Captain John Sheridan, are at a severe disadvantage at this point because one of the renegade ships that was fighting with them has been destroyed. They think they're going to have to surrender when they detect 4 jump points forming right on top of the station and 4 Minbari ships jump out of hyperspace, 3 Sharlin class heavy cruisers and a Whitestar gunboat...
(Note: I'm using some screenplay terms to give an idea of the rythm of the speech. Something in parnthesis is not spoken. It's a direction to an actor)
DELENN: This is Ambassador Delenn, of the Minbari. Babylon 5 is under our protection. Withdraw! (beat) or be destroyed.
EA CAPTAIN: Negative! We have authority here. Do not force us to engage your ship.
DELENN: Why not? Only one human captain has ever survived battle with a Minbari fleet. (beat) He is behind me. You are in front of me. If you value your lives, be somewhere else!