The 2nd amendment is the nuclear option of the citizenry. If the government goes maximum ape and starts "dealing with" its political opponents, the citizenry will revolt. Yes the government has APCs, tanks, and other weapons of mass destruction....but that doesn't stop the random joe with a rifle from putting a bullet between the eyes of [insert politically related individual here].

The chaos and pandemonium that would be caused is immeasurable, and realistically speaking it wouldn't take much of the population to cause an extreme headache for the government. If an issue shows up on the homefront, that is significant enough to require recalling our forces abroad, that will have massive geopolitical ramifications, ramifications that the government really does not want to deal with. Combine this with general unrest driving productivity into the floor and pandemonium throughout, and you swiftly end up in a fall of rome scenario.