Eh comparing one city being destroyed to a whole planet isn't a strong enough parallel. If say the contiguous US was destroyed and all that was left was Hawaii, Alaska, and the US territories (Guam, Virgin Islands, Puerto Rico); America could not operate from any point of strength to enforce it's treaties. The US's dealings with Russia (over Nuclear Nonproliferation, the crisis in Syria, and in the Ukraine), Iran (Isreal conflict and Iran's pursuit of nukes), Iraq, Afghanistan, North Korean etc. The balance of power would shift to much on the planet and America would not be in any position to enforce it's agenda or retaliate for infringed trusts and treaties. Seeing America's weakness, countries who were opposed to America's agenda would presumably move right along with their own agendas without fear of recompense. America's allies would rally to protect it, but with how disengaged Europe and Eastern nations are with the crisis' in the Middle East and the former USSR countries. I doubt they would get involved to support America's affairs in those matters. Bringing this back to Star Trek; yes the RSE would still exist but I don't think they'd be in any position to retaliate on the Fed if Starfleet began deploying cloaking technology on their ships. Losing a planet would be tough, and we don't know enough about the planets that make up the RSE. Are they Romulan colonies or are they Romulan territories annexed through conquest and siege?