“These boots are made for walking, and that's just what they'll do. One of these days these boots are gonna walk all over you.” Nancy Sinatra’s 1966 hit song, “These Boots Are Made for Walking,” became wildly popular with GIs during the Vietnam War, where the lyrics took on a more bellicose meaning.

In discussions of America’s current conflict with the Islamic State of Iraq and the Levant (ISIL)—which, like the Vietnam conflict, Washington does not consider an official war—a phrase heard frequently is “boots on the ground.” It is a direct challenge to those who believe wars can be won by airpower alone. Critics of the current air campaign in Iraq and Syria argue that boots on the ground—the physical presence of soldiers on the battlefield—is a prerequisite to military success. But how many?

There are currently somewhere around 2,000 American military personnel deployed in Iraq, protecting the U.S. Embassy, helping the Iraqi forces coordinate military operations and assisting the air campaign. They have no direct combat role, although some may engage in special operations, such as attempting to rescue hostages. Strategists outside of government have suggested the need for 10,000 or 25,000 American combat troops.

Boots on the ground represent a capability, not a strategy.

Boots on the ground represent a capability, not a strategy. The question is, what would 25,000 American ground forces do that nearly 300,000 Iraqi soldiers cannot do?

They could bolster local defenses in critical areas, reinforcing Iraqi or Kurdish forces that are hard-pressed by ISIL fighters. This is not just a matter of added firepower. Their presence on the ground could also enhance the effectiveness of the air campaign. And with American combat units at their side, Iraqi units might fight harder—or they might fight less, leaving it to the Americans to do the bloody work.

American combat forces could also be used as a mobile strike force to follow up the bombings or destroy concentrations of enemy forces. In this kind of deployment, the combat units would be moved from place to place to exploit opportunities, rather than to hold terrain.

A more ambitious and costlier task for American forces would be to drive ISIL forces out of the cities and towns they now hold. Urban warfare, especially against dug-in defenders, chews up armies. As we have seen on numerous occasions, from the battle of Hue in 1968 to the second battle of Fallujah in 2004, urban engagements can become ferocious fights. More than 13,000 American, British, and Iraqi forces were engaged in Fallujah, and they suffered nearly a thousand casualties.

Controlling territory following the defeat of enemy forces would take far more than 25,000 troops, but clearing cities of ISIL forces, while leaving subsequent mopping up operations and occupation to Iraqi forces (or Shia militias), also risks associating the United States with the vengeance likely to be inflicted upon Sunni fighters and civilians, which the calculated brutality of ISIL has made almost inevitable.

Another possible mission for American combat forces might be to create protected enclaves for refugees.

Another possible mission for American combat forces might be to create protected enclaves for refugees. In 2012, John McCain, Joe Lieberman and four other senators called for the creation of “safe zones” inside Syria, where refugees could find safe havens and anti-Assad rebels could be trained and armed. The senators did call for American combat troops to perform this mission then, and it is not clear how many it would require to so do now.

All of these possible missions are fraught with risk—heavy casualties among U.S. troops and civilians, beheadings of American prisoners, the even greater probability of terrorist attacks abroad and in the United States, the loss of international support. While Americans currently support the bombing campaign, hard fighting on the ground could quickly reverse that support.

The United States initiated the bombing campaign to prevent the consolidation and further expansion of ISIL and preempt potential future terrorist attacks on the United States. The deployment of ground forces could accelerate the achievement of that mission, but could just as easily exacerbate the situation.

As we learned in Vietnam, boots on the ground, a phrase that was popular during the buildup of American forces there, is no guarantee of success. Before embracing boots on the ground as a strategy, it is essential to be clear about what they are going to do, what they may require and what risks may be entailed.