But here's the thing, if the Romulans know for sure that the Federation's response to an act of war is, well, actually going to war, then maybe they won't have to keep testing the Federation.
If a foreign power attacked your nation and killed your fellow citizens, would you not expect your government to respond with the full resources of your country's armed forces? (e.g Pearl Harbour)
Not necessarily.
There is a such thing as a proportional response. I'm sorry, but a couple of outposts are not worth the costs of an interstellar war if the immediate threat can be neutralized.
The West Wing argues it fairly well. The set-up: The President of the United States, Jed Bartlet, is about to go on national TV to announce that the U.S. has destroyed the headquarters of Syrian Intelligence in retaliation for Syria's having shot down a U.S. Air Force plane. The President was friends with a man aboard, and he wants to launch a full-scale war in retaliation for the attack. Leo, his chief of staff, is talking him down.
Man that was a good scene. I rmb watching that episode years ago when it first aired.
Anyway, what's a proportional retaliatory strike is debatable. You and a few others think that the destruction of the Bird of Prey is sufficient. I disagree.
I think if the Romulans attacked a couple of outposts in the middle of nowhere (like Amargosa Observatory and one of those little relay stations with like 2 ppl on it), I'd agree that pursuing and destroying the Romulan ship responsible is sufficient reprisal.
But I think the loss of a strategically important outpost that is clearly in Federation or allied space is sufficient cause for war (Neutral Zone Outposts, Deep Space 9) I also think that Federation security has to be compromised at least temporarily until those outposts are rebuilt.
The map in "Balance of Terror" showed seven outposts along the Neutral Zone. Three of them were destroyed. If North Korea crossed the DMZ and wiped out 3/7ths of the US-South Korean troops guarding the DMZ, I think they would go to war over that too.
Also, the West Wing episode is slightly different in that the Federation is not the only superpower. The United States clearly has much greater military strength than Syria. But Starfleet is probably on par with the Romulan military. The Federation can't afford to show weakness to enemy like that.
When Picard dropped his shields in "The Enemy", he warned Tomalok that destroying the Enterprise would plunge the Empire into war. The Romulans have to believe that the Federation is willing to go to war over the destruction of a single starship, the Enterprise-D, just as they have to believe that the Federation would go to war over the destruction of Neutral Zone outposts.
Just as the Klingons had to believe that capturing or destroying DS9 would plunge their Empire into a war with the Federation at the same time they were fighting the Cardassians.
Tomolok believed it. So did Gowron. Both of them did not carry through with their attacks.
But the Romulans in the Tomed Incident probably didn't. Neither did the Cardassians planning to invade Minos Korva.