Why did the Federation and the Romulans never ally?

[Eternal_Freedom]

Based on "Balance of Terror" it would seem the Romulans are completely surrounded by the UFP. As Spock explains, you have the "Romulan Neutral Zone" agreed by treaty with Earth and her allies, which separates Romulus and Remus from "the rest of the galaxy."

Sounds very much like the Romulans are surrounded in that instance. Of course, in "The Enterprise Incident" we learn that the Romulans and the Klingons had some kind of alliance, which may well have allowed the Romulans to expand in that direction (the Beta Quadrant) for some time.

[Baffalo]

Based on "Balance of Terror" it would seem the Romulans are completely surrounded by the UFP. As Spock explains, you have the "Romulan Neutral Zone" agreed by treaty with Earth and her allies, which separates Romulus and Remus from "the rest of the galaxy."

Sounds very much like the Romulans are surrounded in that instance. Of course, in "The Enterprise Incident" we learn that the Romulans and the Klingons had some kind of alliance, which may well have allowed the Romulans to expand in that direction (the Beta Quadrant) for some time.

Reading Memory Alpha about the RNZ shows that the Neutral Zone was redefined in the Treaty of Algeron (ENT: These are the Voyages...). This means that the RNZ is somewhat flexible, and makes lots of sense. It gave the Romulans a bit of room to expand and removed any lingering questions about where the border lay.

That sounds good from the perspective of the "Romulans are completely within the Federation". But I don't see that as being true for a couple of reasons. One, the Federation went to war with the Romulans before, which set the border. Two, the Federation and Romulans are on somewhat equal footing, so the Romulans develop their warp capability at roughly the same pace. Third, even if you accept a birth rate double or even triple that of the Romulans, it would take quite a bit of effort to completely surround the Romulans. I think the Neutral Zone is just where the Romulan and Federation border is officially put, getting more vague the further out you go from the established border. It's no different than the established border between the US and Canada... even though both sides have clear claims to an area, establishing an arbitrary point avoids conflict later, and it's one that's easy to identify for both sides.

[Darth Fanboy]

If Romulus is as close to the Border as some maps depict, then a mural with Romulus's position in the galaxy would reasonably include the border