America turned to the private sector for personnel that its all-volunteer military could not muster. In Iraq half of the personnel in war zones were contracted, and in Afghanistan it was closer to 70 percent. America may fight future wars mainly with contractors.

Now others are following America’s lead. Nigeria hired hundreds of mercenaries to fight Boko Haram. Russia is allegedly using them in Ukraine. And oil companies and humanitarian organizations are turning to private military companies to protect their workers and property in dangerous places, and there is an argument that the United Nations should use this industry to augment thinning peacekeeping missions.

Private force isn’t a new phenomenon. Contract warfare was the norm in the Middle Ages. Like today, for-profit warriors were called condottiere (“contractor” in old Italian). They usually fought for the highest bidders: kings, city-states, rich families, even popes.

The problems associated with private force were solved when states began monopolizing the market and put mercenaries out of business. They created large national armies accountable to governments and bonded by patriotism, rather than cash. This process took centuries, but is now unraveling.

Few would welcome a new unbridled market for force, yet it is already developing. The industry continues to proliferate as new consumers seek security in a deeply insecure world. And mercenaries are less expensive than standing armies, just like renting a car is cheaper than owning one. The Congressional Budget Office found in 2008 that Blackwater cost 10 percent less than a comparable army unit in wartime Iraq, and a private force costs nothing in peacetime because its contract can be terminated.

The question now is how to minimize the risks of private armies. Some companies have proposed self-regulation, but that proposal lacks teeth and is dependent on firms confessing their own crimes, which is bad for business. Nor can it be externally regulated; strict laws will only drive these firms offshore or underground.

The solution is to use market power. Superclients, like the United States or the United Nations, could shape best practices by rewarding good firms with profitable contracts and withholding them from bad firms. America or the U.N. could establish a licensing and registration regime that all industry members must observe in order to be eligible for contracts. This would include clear standards for training and vetting members and transparent mechanisms for oversight and accountability.