After more than a decade of war in Iraq and Afghanistan, America’s most profound legacy could be that it set the world order back to the Middle Ages.You're probably thinking of the President's Iraq policy allowing a 7th century quasi-government to seize control of much of the Levant, while playing along with Iran's nuclear ambitions in a way likely to secure the theocracy's position among the nations of the world, while making it ever more likely that we'll see a renewed war in the Holy Land.
What he's actually thinking about is the way private contractors remind him of 14th century mercenary armies. There's a strong argument for privatizing our response to groups like ISIS, though, which is that small private firms can offer a short decision chain similar to our enemy's. We don't lose the capacities provided by formal government armies, but those have Schumpeter's disadvantage in dealing with highly adaptable small forces. This is why Marx was wrong about capitalism leading inevitably to monopoly: small competitors lack the economies of scale, but they can often pick off pieces of the larger businesses because their ossified decision-making chains take too long to compete.
That works with 'monopoly on force,' too. As long as contractors are employed by legitimate governments and held accountable according to the ordinary laws of war, there's no reason they should not be used to deal with ISIS-type threats. It's my sense that it's actually much easier to hold private firms and their employees accountable using government mechanisms than for the government to hold itself and its own accountable. There's no sense of protecting one's own that would derail prosecutions, and it's very easy to cancel a contract.
Besides, the 14th century was the during good part of the Middle Ages.