A display of the 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. (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 Founders' homeworld. (DS9: "Improbable Cause")

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. (DS9: "In the Pale Moonlight")

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. (DS9: "In the Pale Moonlight")

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…" (DS9: "Strange Bedfellows")

See also

The script for "In the Pale Moonlight" states that the display on screen is the "Cardassian/Romulan border". [1]
The Romulan Carraya system may have been located near the border, since it was stated to be "not far" from Deep Space 9 in "Birthright, Part I".
According to the Star Trek: Star Charts and Stellar Cartography: The Starfleet Reference Library, the Cardassian Union does not have a border with the Romulan Star Empire. As the star chart seen in the Star Trek: Picard episode "Maps and Legends" was based on these reference works, it shows the Cardassisn Union as not having a border with the Romulan Star Empire and the Carraya system being quite distant from Deep Space 9.
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