The Federation would win because the Tal Shiar fleet was destroyed at the outbreak of war and by the time the Allied Fleet attacked Cardassia Prime attempting to break the blockade the Romulan Fleet had taken substantial casualties, in battle and over the long campaign had caused the Romulans to take the blunt of casualties. And in 2287 according to Spock Prime Romulus is destroyed so i think it's pretty much over right there.
1. The Tal Shiar fleet was destroyed more than two years
before the outbreak of the Dominion War.
2. There's no on-screen evidence that the Romulan fleet had taken heavy casualties. Only that the flagship was destroyed.
3. That Romulus is destroyed doesn't mean that the RSE would suddenly cease to function. The majority of their fleet would still be intact, as it's unlikely most of their vessels would be stationed anywhere near Romulus at the time of the supernova.
4. Is it really too trouble for people to use proper grammar and punctuation? I realize that not everyone here is the same age or speaks English as their first language, but it's disturbing that people can't go to the trouble of typing sentences and phrases that make sense.