The Mars Defense Perimeter was part of the Federation planetary defenses of the Sol system. It was designed to protect the inner planets of the system, and specifically Earth, from intruding hostile starships. The defense perimeter consisted of a series of sentry pods.
In early 2367, an attacking Borg cube crossed the planetary defenses between Jupiter and Mars. It was able to easily break through the defensive lines, destroying the ships sent to intercept it within a matter of seconds. (TNG: "The Best of Both Worlds, Part II")