This one from DS9: Rocks and Shoals has always stuck with me.

Sisko: And when they’ve reached this point, we’ll have them in a crossfire.

Gordon: They won’t have a chance.

Garak: That is the point. In case you’ve forgotten, we’re in a war.

O’Brien: There are rules, Garak, even in a war.

Garak: Correction. Humans have rules in war. Rules that tend to make victory a little harder to achieve, in my opinion.

Gordon: So we just shoot them down?

Neeley: They wouldn’t hesitate if the situation was reversed.

Nog: But we’re not the Jem’Hadar. It is our duty to —

Sisko: THIS ISN’T A VOTE!

(stunned, everyone turns and looks at Sisko)

Sisko: The decision’s mine. And Mister Garak is right. We are at war. Given the choice between us or them, THERE. IS. NO. CHOICE. Let’s move out.