The Benzar defense perimeter was a planetary defense system that defended the planet Benzar. (The Dominion War Sourcebook: The Fires of Armageddon)
During the Dominion War the Benzar defense perimeter and the Starfleet Eleventh Fleet was unable to prevent a Dominion invasion of the planet. The Dominion were able to capture several of the defense platforms and used them against the Starfleet Fourteenth Fleet that attempted to dislodge the Dominion.
The perimeter was reinforced by Dominion ships and technology during their hold of the planet. Using cloaking devices the Romulan Star Navy was able to breach the perimeter and attack it from behind during the liberation of the planet.