Presumably, something happens after the original Star Trek (I don't remember seeing any TNG eps, but I read some books), where humans (terrans) become second-class citizens. They were known for using incredibly powerful weapons that everyone feared.

I remember one DS9 ep where the Klingons rule the world, but some human rebels get together with Sisko to "create" the Defiant. I believe that was the only instance where the Negh'Var class Klingon battlecruiser was shown (huge ship). It sure got it's @ss handed back to it by the Defiant.