The Federation's line in the sand against the Romulans.
Established after the Earth
wars of the 22nd century, the Romulan Neutral Zone
was a band of space that was absolutely demilitarized. The neutral zone provided both states with a buffer area that kept tensions low by giving plenty of time to see an attack coming. This large area of open space that had to be crossed before any attack against the Federation
could be made may have influenced the Romulans to develop cloaking devices
for their fleets. Ironically, after one Romulan violation of the Neutral Zone, the Federation signed a treaty that forbade their own development of similar technology.