The Mars Defense Perimeter was a Starfleet defensive program set up to defend the inner Sol system, specifically Earth, from attack from outside the system.
During the Borg Incursion of 2366-2367, a number of ships comprising the Mars Defense Perimeter were easily destroyed by an approaching Borg cube. (TNG episode: "The Best of Both Worlds, Part II")
In 2371, the USS O'Keefe was assigned duty on the Mars Defense Perimeter.
Command of the Mars Defensive Perimeter was based at Utopia Planitia Control. (CoE eBook: Remembrance of Things Past, Book I)
In 2375, the Breen broke through the Mars Defense Perimeter on their way to attack Earth. (Last Unicorn RPG module: Star Trek: Deep Space Nine Core Game Book)