Now, as President Obama prepares to hand off combat operations in Iraq, Afghanistan, Syria, and elsewhere, to his successor, he’s also bequeathing a way of war that relies on large numbers of guns-for-hire while, at least formally, restricting the number of American “troops” sent overseas. Since 2009, the ratio of contractors to troops in war zones has increased from 1 to 1 to about 3 to 1.

Private military contractors perform tasks once thought to be inherently governmental, such as raising foreign armies, conducting intelligence analysis and trigger-pulling. During the Iraq and Afghanistan wars, they constituted about 15 percent of all contractors. But don’t let the numbers fool you. Their failures have an outsized impact on U.S. strategy. When a squad of Blackwater contractors killed 17 civilians at a Bagdad traffic circle in 2007, it provoked a firestorm in Iraq and at home, marking one of the nadirs of that war.

Contractors also encourage mission creep, because contractors don't count as "boots on the ground." Congress does not consider them to be troops, and therefore contractors do not count again troop-level caps in places like Iraq. The U.S. government does not track contractor numbers in war zones. As a result, the government can put more people on the ground than it reports to the American people, encouraging mission creep and rendering contractors virtually invisible.

For decades now, the centrality of contracting in American warfare—both on the battlefield and in support of those on the battlefield—has been growing. During World War II, about 10 percent of America’s armed forces were contracted. During the wars in Iraq and Afghanistan, that proportion leapt to 50 percent. This big number signals a disturbing trend: the United States has developed a dependency on the private sector to wage war, a strategic vulnerability. Today, America can no longer go to war without the private sector.

Why did this happen? During the wars in Iraq and Afghanistan, policy makers assumed a quick and easy victory. As former Defense Secretary Donald Rumsfeld said in 2002, the Iraq War would take “five days or five weeks or five months, but it certainly isn't going to last any longer than that.” When these wars did not end in mere months, the all-volunteer force found it could not recruit enough volunteers to sustain two long wars. That left policy makers with three terrible options. First, withdraw and concede the fight to the terrorists (unthinkable). Second, institute a Vietnam-like draft to fill the ranks (political suicide). Third, bring in contractors to fill the ranks. Not surprisingly, both the Bush and Obama administrations opted for contractors.

Today, 75 percent of U.S. forces in Afghanistan are contracted. Only about 10 percent of these contractors are armed, but this matters not. The greater point is that America is waging a war largely via contractors, and U.S. combat forces would be impotent without them. If this trend continues, we might see 80 or 90 percent of the force contracted in future wars.