We've seen the attack on Mars, there were no Romulan Warbirds involved. And besides, even cloaked there's no way a Romulan Warbird could make it all the way to the capital system of the Federation undetected, let alone 218.The burden of proof is on the Romulans to show that those 218 warbird crews weren't also involved with Mars, which was very much Federation territory.
And like it or not, being suborned by an intelligence agency does get the crews of those warbirds and the Romulan government off the hook. Like the joint Cardassian-Romulan fleet that attacked the Founder's homeworld in DS9's third season. They were under the control of the Tal Shiar and the Obsidian Order, which the Dominion considered satisfactory enough to allow the Cardassians to later join the Dominion and the Romulans to sign a non-aggression pact. If the Dominion were willing to forgive two governments for the actions of their intelligence agencies, I think the Federation will also.