A display of the Cardassian-Romulan border
The Cardassian-Romulan border was the division between the Romulan Star Empire and the Cardassian Union. Part of the border ran through the Glintara sector, while the Unefra system was located on the Cardassian side of the border. According to a holographic projection of Weyoun, Romulus could be reached by Dominion ships within one day of crossing the border. (DS9: "Improbable Cause", "In the Pale Moonlight")
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 Founder's homeworld. (DS9: "Improbable Cause")
In the forged holorecording created by Grathon Tolar in 2374, the planned Dominion invasion of Romulan space began with the advance of the 23rd Jem'Hadar Division across the Cardassian-Romulan border via the Glintara sector, with the Cardassian Fourth Order protecting their flanks. (DS9: "In the Pale Moonlight")
During the Dominion War, Jem'Hadar forces frequently violated Romulan space 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, they began by striking 15 outposts along the Cardassian-Romulan border, opening up a second front. (DS9: "In the Pale Moonlight")