The Iraq war was a turning point in this area, actually. Stretched between two active conflicts far from each other, and having re-structured the US military as a smaller army of highly-specialised troops, there were not enough boots on the ground to maintain security across the whole country. Enter unregulated mercenary armies.

Sure enough, the US is re-learning why mercenary armies have always been a bad idea throughout history.

Hardly surprising, given the size and propensity of the US to engage in armed conflict with organized fighting forces. (Who else on the planet could possibly be a bigger customer than the US?)