The entire Federation would have most certainly struck back regardless of Shinzon's next actions, since the existence and safety of the Federation depends on its commitment to the protection and well-being of all its members, much less a key member like Earth. Without this commitment, neighboring superpowers would be free to invade any Federation world without fear of reprisal, and there would be really no purpose to being a Federation member.
However, Shinzon only had a single ship. It was an impressive ship capable of committing acts of genocide on a planetary scale and with state-of-the-art cloaking technology, but it would still be no match against the collective fleet of the Vulcans, Humans, Andorians, Benzars, Bolians, and numerous other Federation members. And that's not even taking the Romulans and Klingons into account.
So I wouldn't necessarily call it a war so much as a hunt for a war criminal or rogue warbird. Shinzon is a brilliant enough tactician to probably destroy a considerable number of Federation and Romulan vessels, but without the backing of the Romulans, he really has no chance, never mind the fact that he was dying of cellular breakdown.
So as soon as Shinzon's Romulan supporters backed out, the Federation was no longer at risk, just Earth. If he had unleashed his weapon on Earth, it would undoubtedly change the course of history, but he wouldn't have gotten much further than that.