Then, why does the Federation ships (exploration vessels) seem to have so little trouble defeating the Klingon and Romulan ships, ships specifically designed for combat. What do others out their think?

I mean: Look at the Defiant and the Prometheus. The Defiant can rip apart Dominion Bugs in 2 seconds flat! And the Prometheus destroyed a Warbird (with some help of a Defiant class).

I would love to have seen a REAL Federation battlefleet engage the Dominion... not to mention a Borg cube.
Have you ever imagined what it would look like if they shot even ONE tri-cobalt device at a Borg cube??

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The Miranda Class model is not a kitbash, it is a bashkit.

A rather impressive nothing, as the slow moving demolition device impacts harmlessly on the surface of the cube.

[post-Voyager Conspiricy]

A slightly impressive something, as the subspace effects of the weapon might possibly wreak a little damage, but I would imagine the Borg would get better, unless the device works equally well against fully powered and shielded targets.

In regards to the original post: we've never really seen a large scale battle between those three powers. Certainly, if we go by offscreen and spoken evidence, the Klingons gave the UFP a run for its money during their brief pre-Dominion War war.