In the TOS episode "Balance of Terror," the Romulan flagship crosses the Neutral Zone to commit a dastardly sneak attack. It annihilates several Federation outposts before Kirk finally kicks ass.
Why the Romulans opt for destruction instead of peace remains largely unanswered. The episode establishes an indecisive Earth-Romulan War from "a century ago," duration unclear. Even granting a decade's worth of margin this still plants the war directly in the Series V timeframe. We might expect Captain Archer to know a certain "Captain Stiles," one combatant identified in the conflict.
After the war, a treaty was agreed to which established the Neutral Zone. The relatively primitive technology of the day left this treaty negotiated by subspace radio alone, leaving no clues for Kirk's time as to the nature of their adversary. It turns out that the Romulans and the Vulcans are physically-related species, whereas the Vulcan race has evolved to embrace IDIC and the peace it affords.
So it could be that the Vulcans just aren't talking. Note that Spock characterizes both Terrans and Romulans as having an "aggressive, colonizing phase" even as he describes such a phase as part of Vulcan's past. Spock really doesn't even seem surprised when he first acquires a "fix" on the Romulan bridge (through a cloak, leaving no means for sensors to track - just what's that main viewer made of?!), and it's reasonable that he knew what he would find there.
Are the Vulcans aware of their nasty brutish cousins, and doing what they must - no more of course, only as little as necessary - to protect the innocents among their path? Were their "early" efforts so obviously ineffective that they felt compelled to turn outwards "later?" We should sniff out T'Pol's comments carefully regarding her expectations in exploring uncharted space.
(Twice the shame then, should future plot twists involve a needless "temporal villain" while this rather satisfactory villain has already been prepared. Perhaps the Romulans "trick" the Suliban using unknown technology - they seem to come up with such stuff pretty frequently!)
A Lieutenant Stiles serves the NCC-1701 bridge, nursing an ignorant antipathy towards Vulcans. Against this Kirk has a remedy: "Leave any bigotry in your quarters ... there's no room for it on the bridge." Unlike the Romulan, Kirk does not seek "the glory of the kill" (as if there could possibly be glory in killing). His method for dealing with the Godless, cowardly sneak attack is simple. Exact justice from the responsible - but don't use hate to try and defeat hatred.