The Mars Defense Perimeter was a planetary defense system in the Sol system designed to protect the inner planets, including Earth from attack. The perimeter was made up of attack ships and Mars Defense Perimeter platforms.
A second Borg attack in 2373 was again able to penetrate the perimeter. (The Dominion War Sourcebook: The Fires of Armageddon)
During the Breen Attack on Earth in 2375, the Breen were able to demolish the Mars Defense Perimeter and the Starfleet Third Fleet in two hours. (The Dominion War Sourcebook: The Fires of Armageddon)
In 2410, Starfleet prepared to engage the Iconian Heralds at the Mars Defense Perimeter, but Rear Admiral Zandra Taitt was able to prevent the incursion altogether at the cost of her own life by diverting the invasion fleet into a black hole. (Bait and Switch: Beat the Drums of War)