Cardassian-Romulan border
The Cardassian-Romulan border was the division between the Romulan Star Empire and the Cardassian Union. The Unefra system was located on the Cardassian side of the border, while the Glintara sector was located between the border and Romulus. ()

In 2371, a Cardassian informant told Odo that a number of Romulan ships, believed to be warbirds, were detected near the border, along with troop movements and increased communications activity. The informant admitted the Cardassian authorities considered this a possible prelude to a Romulan invasion, but that this made little sense. Central Command was unaware that the buildup was actually part of a top secret, joint Obsidian Order-Tal Shiar operation to launch a preemptive first-strike against the Founders' homeworld. ()

During the Dominion War, Jem'Hadar forces frequently violated Romulan territory by crossing the border in order to attack Federation targets near the Romulan Neutral Zone. The Romulans ignored these transgressions due to their nonaggression pact with the Dominion. However, when the Romulans finally entered the war on the side of the Federation Alliance in 2374, they began by striking 15 outposts along the Cardassian-Romulan border, opening up a second front. () 

In the forged holo-recording created by Grathon Tolar in 2374, the border was relatively undefended since the bulk of the Romulan military was deployed along the Romulan Neutral Zone. The Founders therefore predicted, that in the planned Dominion invasion of Romulan space the Dominion could be on Romulus in less than three weeks. () 

Following the alliance between the Dominion and the Breen Confederacy in late 2375, Breen Thot Gor shared an "interesting observation" with Weyoun regarding the Dominion forces along the Romulan border, which was in turn shared with Damar. According to Gor, via Weyoun, "In his view, we have failed to take advantage of several weaknesses in their defenses. […] For instance, [the] Romulan colony in the Unroth system is ripe for attack. Their long range sensor array is being repaired…" ()

See also  
 Romulan-Klingon border
 Romulan Neutral Zone

Cartography
Cardassia
Romulus