system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Trek] Why can't the United Federation of Planets not completely bulldoze other powers such as the Romulans or Klingons in a war?
gpt
In the TOS era, there was rough parity between Alpha Quadrant powers, with the Romulans probably being fewer in number but more tactically effective craft (plasma torpedos and cloaks), the Fed (mostly due to the Tellurian, Vulcan and Andorian presence) in the middle of the numbers vs. tactical quality and the Klingons with the largest forces. Gorn were medium threats with greater sophistication and genetically modified warriors. Major conflicts during this period were likely to stalemate, so the Fed can't just bully them while Pike and Kirk were around.

By TNG period, the Fed is enormous in volume and population but very spread out. At this point, non Warship Fed vessels were equivalent to warships of equivalent size, and peer conflicts would go the Fed's way as long as there was sufficient ramp up time for a wartime economy. The only real threat among peer or near peer adversaries was the Dominion, a civilization that occupied and controlled the greater part of a whole Quadrant. While costly, active wars with any of the major powers would end in the Fed's favor, but the Fed is decentralized and inherently non aggressive in its expansion and hegemony\*.

This is because, as Quark points out in DS9, the Fed does not need to be aggressive in its expansion. It incudes large volumes of undeveloped space, internal and external frontiers of expansion, positive population growth, increasing incorporation of neutrally aligned worlds, positive trade balances with other powers, and increasingly advanced technology as a product of both its science/exploration focus and the greater cooperation between its member states. The only real time we see and existential threat it's from the Borg (see the Farming Theory for more), the Dominion and through time shenanigan's the Klingon's in Yesterday's Enterprise.

So there's a practical reason they don't (it's unnecessary) and a political/philosophical one I'll briefly expand on here: The Fed considers it's reputation for impartiality and neutrality an asset. Rather than shoot first, they have calculated that the peaceful stance gains them more, in a fundamental strategic sense, than the types of geopolitics we see from other powers (and here on earth in Great Power struggles). To them, the benefits internally and externally from being a responsible power outweigh the potential for them to militarily expand their territory.

\*The Fed *greatly* expanded compared to other AQ powers from TOS through to the end of VOY; there approach not only seems to work, it only fails when the Fed ossifies in the 30th century, there's a vicious Time War *and* theres some weird shit with a kid in a solid dilithium asteroid.

tl;dr- The Fed *is* steamrolling the rest of the major powers in the setting, but they're doing so through largely cultural and economic methods and prefer to exercise soft power whenever possible because there strategic stance is such that they benefit more from being moral. Given the presence of several long lived species in the core Fed polity (Vulcans and Andorians) this long term strategy seems to work for them.