The Vulcans were one of the founding members of the Federation and chose diplomacy rather than empire building as their preferred method of developing relations with other species - after making a successful and legal ‘first contact’ of course. As a side note, I am curious if Earth was the first (or last) species that the Vulcans made first contact with? Are there any canon or non canon references or records to Vulcans making this form of contact with other worlds? Were the Vulcans ‘alone out there’ before finding Earth via that infamous and often contorted (Through a Mirror, Darkly, First Contact, Metamorphosis) Zephram Cochrane saga? The Vulcans also seemed to follow the prime directive to the latter, making sure that Earth (and hopefully anyone else that their sensors had detected warp signatures from) had developed warp drive before showing us their spaceship. Had the Vulcans developed a prime directive before the forming of the Federation, was this seemingly Vulcan rule transferred in to the Federation charter’s on the forming of the Federation?
The Vulcans never expanded beyond their home world through colonisation from what we see depicted on screen.
Or did they….
The Vulcan species come across as being very introverted. The Romulans (going back to the original posters post, sorry to go off topic) as depicted in canon on the other hand seemed to be quite the extroverts; they left Vulcan and after colonising Romulus as their new home planet (damn Surak and his teachings!) continued to expand and create an empire of many worlds which spanned a good portion of the Beta Quadrant. We can assume that they did this without following the same laws, directives and logic as their Vulcan ancestors. First of all, they must have colonised and conquered Remus, I’m guessing that it was an uninhabited planet at the time? The Remans are a bit like what the Aenars are to the Andorians from what I can gather and not a subjugated slave species? Obviously the Romulans wanted Remus under their influence as it is rich in natural resources in a very similar way as to how the Klingons needed Praxis, though Praxis was only a moon… Remus was a twin planet so as well as being in a close proximity and controlling the tides it was also capable of supporting a hefty population (I’m assuming Remus was an M class planet like Romulus was?).
So after building this great ‘Romulan empire’, why would they want to invade little Vulcan? Are the Romulans not happy with what they have achieved and what they have built and become?, separate from the conflicts and devastation of the past?
I think that I remember this storyline… Unification.
Maybe the Romulans wanted to invade Vulcan because they had historical ties with the planet? They are literally a long lost and disconnected family.
The Romulans had achieved an empire, the Vulcans had achieved a collaborative Federation in partnership with other species, but perhaps the Federation was not to the Romulans liking as they didn’t like how their ancestral home was slowly beginning to be influenced by external, yet benign, forces… even militarised to some extent, so rather than letting their ancient home land continue to be absorbed in to what they see as being out of their influence (they maybe always wanted to return and reclaim it), they decide to send a secret invasion. Again, if I remember correctly, this invasion fleet was disguised as civilian transports? I think that there were several pew pew’s and a touch of mild peril but no epic space battles.
I think that the Romulans should have approached this in a different way.
Rather than seeing Vulcan as a lost ancestral home, something to take back one day when they have developed enough power… perhaps they should think of Vulcan as being almost like a planetary ‘embassy’ or mutual ground for both the Federation and Romulans. A location where there is a shared history and a lot at stake… a common ground from which to develop mutual relations.
Why would anyone destructively invade their ancestral homeland? Potentially killing their distant cousins and friends?
So, back to the original posters question. If the Romulans would have invaded Vulcan, it would have broken the treaty of Algeron and therefore be considered as an act of war against the Federation.
Ukraine is not a member of NATO (the Federation?). Imagine if Vulcan was Ukraine, and the Romulans were Russia.
That is a space battle that no one wants to see.