Both Max Boot (“More Small Wars,” November/December 2014) and Rick Brennan (“Withdrawal Symptoms,” November/December 2014) provide insight into what the United States did wrong at an operational level in Iraq. Boot’s precepts for doing better in the next counterinsurgency are sensible, even if some of them would require a higher tolerance for casualties, and Brennan’s arguments about the errors the United States committed in Iraq from 2010 to 2012 generally ring true to me, as one of the people making some of those mistakes.

But Boot’s and Brennan’s arguments rely on a flawed assumption: that if only the United States had waged counterinsurgency properly, it could have succeeded. If Washington’s original goal was to transform Iraq such that Baghdad could govern competently, quell the country’s insurgency, and develop functional, Western-style institutions, counter­insurgency was destined to fail—just as the United States failed in Vietnam, Somalia, and Afghanistan. The blame lies not with poor implementation but with the strategy itself.

Counterinsurgency, which General David Petraeus described in The U.S. Army–Marine Corps Counterinsurgency Field Manual in 2006, calls for a three-legged approach known as “clear, hold, and build”: push insurgents out of a designated area, prevent them from returning, and build local institutions that help the population move forward. The U.S. military is capable of implementing the first two legs, since they are primarily military in nature. But it runs into problems with the open-ended nature of the third. The military can enlist civilian U.S. government agencies to provide very limited assistance, but those bodies, too, have spotty records when it comes to implementing reform and reconciliation, even in countries at peace.

The “build” leg is indeed ambitious; it involves developing competent local governments and security forces capable of replacing U.S. forces. Yet here the deck is stacked against success. The United States tends to commit troops to counterinsurgency missions only in the absence of a friendly central government (think of Afghanistan in 2001 and Iraq in 2003) or when one is hopelessly outmatched by insurgents (think of Vietnam in 1965). In such situations, U.S. forces face an uphill battle and are bound to incur heavy losses.

It gets worse: U.S. ground troops, as opposed to military advisers or air forces, almost always generate negative reactions among developing-world populations that are suspicious of neocolonialist intrusions, no matter how carefully the soldiers avoid inflicting civilian casualties or how openly parts of the population greet them. Moreover, neighboring states, alarmed by the prospect of U.S. forces on their borders, often have an interest in supporting the insurgents. North Vietnam and China did so in South Vietnam, Pakistan did so in Afghanistan, and Iran and Syria did so in Iraq.

Such resistance Americanizes the costs of the war, as U.S. commanders assert increasing control over operations to deal with the growing threats. And even when the military simply “holds,” the price in both blood and treasure is substantial. To justify such high costs, “build” operations become more grandiose, ever more a reflection of how the United States thinks a humanitarian, efficient, economically viable state should look. Host-country officials, dependent on Washington for their survival and profiting from its largess, go along super­ficially. But critical reforms—such as the three separate anticorruption mechanisms I helped establish in Baghdad during my time there—fail to survive without constant U.S. attention. For as long as U.S. forces remain, the stabilization effort runs on, buoyed by a flood of suspiciously positive metrics.

A U.S. soldier in Kandahar counts down the days until he goes home, April 5, 2008. oran Tomasevic / Reuters

Once U.S. forces leave, a situation similar to that seen in Iraq after 2011 is all but inevitable. Some reforms stick. Iraqis, for example, have maintained a stubborn adherence to voting and constitutional norms. Others fall by the wayside. If no real threat appears, the state stumbles on; if one emerges, as the Islamic State of Iraq and al-Sham (ISIS) did in 2014, disaster follows.

Boot and Brennan seem to suggest that if the United States had kept troops in Iraq long enough, it could have avoided such a fate. But that kind of argument, applied to counterinsurgency writ large, has never been fully tested for a simple reason: sooner or later, the American public always grows weary of such campaigns and pressures Washington to end them. Boot claims that U.S. presidents have underestimated the American people’s “impressive patience” for such endeavors, describing the resistance to the Iraq war as politically significant only in 2006–7, “when U.S. fatalities in Iraq reached over 100 a month and the war looked lost.” But that is like saying that opposition to the Vietnam War mattered only at its height, in 1968–70. It misreads the more general popular mood, which was ugly throughout both wars. In each case, the White House assuaged public doubts with not only battlefield achievements but also assurances that the conflicts would soon end. President George W. Bush boldly bucked public opinion and escalated the Iraq war in 2007, just as President Richard Nixon did in Vietnam when he sent troops into Cambodia and Laos in 1970–71. But Bush, like Nixon, accompanied his bellicose action with an emphasis on turning the conflict over to the host government. He began withdrawing combat troops shortly after the last surge brigades were deployed. He also promised Iraq’s leaders that the United States would withdraw all its forces by the end of 2011. Had either president not reassured the public of an imminent withdrawal, the broad resistance to both wars in those critical periods would have exploded.

A U.S. Marine from Lima company passes an Afghan man during patrol in Helmand province, April 8, 2010. Asmaa Waguih / Reuters

Whatever changes the United States wishes to see in the world, it must remember that the U.S. military exists to complete military missions, such as defeating Nazi Germany or driving Iraq out of Kuwait. When political leaders give the Pentagon broad goals of social transformation in the guise of “Phase IV stability operations,” they undermine support for even legitimate, low-cost military missions, such as an air campaign in Syria. Counterinsurgency was a recipe for defeat and retrenchment in the recent past, just as it was in the 1970s and will be again.

What, then, should U.S. policymakers do when faced with an insurgency? If possible, Washington should respond by backing friendly local forces. If not, it should accept the consequences of a victorious insurgency, contain its spread, and protect critical allies. But to embark on another U.S.-troop-centric counterinsurgency mission would do an injustice to the fine men and women who serve in the U.S. military.