When conflict rears its ugly head around the world, there is usually a call for the United States to “do something.” One option that is frequently mentioned is the no-fly zone. The United States and its allies enjoy a significant advantage over most potential adversaries in the air. No-fly zones, therefore, are attractive due to the perceived lower cost and risk when compared to other options. Despite this, setting up a no-fly zone is anything but a “no brainer.” Depending on the circumstances, there may be steep costs and unseen risks. This short primer is intended to introduce readers to the way no-fly zones really work.

What is a No-Fly Zone?

A no-fly zone is airspace designated as “off limits” to flight-related activities. There needs to be an explicit policy concerning which actions that are prohibited in the zone, and this should be communicated clearly. In addition, there must be some form of punishment in response to violations. Typically, this involves friendly military aircraft intercepting violators and escorting them away, forcing them to land, or shooting them down.

Why Would We Establish a No-Fly Zone?

There are many reasons, but the most likely is to stop adversary aircraft from attacking or harming people on the ground—including friendly forces and civilians. In addition, establishing a no-fly zone can negate an adversary’s military advantage, put pressure on an adversary to make concessions, give hope and relief to people who have been attacked, weaken and demoralize an enemy air force, or serve as a prelude to invasion. Furthermore, a no-fly zone could be combined with other actions—such as a naval blockade or closing of a border—to stop the flow of goods and people into an area, further weakening an adversary.

When is a No-Fly Zone Likely to be Effective?

In most circumstances, a no-fly zone is effective only if the adversary has significant air forces. If the adversary’s air force is too strong, however, it will be very difficult to set up an effective no-fly zone. Therefore, a no-fly zone is most likely to be effective against an adversary whose air forces are substantial, but not too substantial.

Moreover, a no-fly zone can alter the balance of power on the ground. If one side relies heavily on airpower and the other does not, a no-fly zone can level the playing field and bolster forces that have been subjected to invasive observation or painful attacks. Conversely, if airpower is not an important factor, it’s unlikely that establishing a no-fly zone will do much (unless the mandate is extended to destroying ground targets).

What are the Requirements for a No-Fly Zone?

In the past, the United States has sought international approval when establishing a no-fly zone, usually from the United Nations Security Council. This provides a form of legitimacy. Without it, the legal basis for the no-fly zone will be questioned, as will our commitment to international norms.

The next requirement is to decide upon rules of engagement—the set of guidelines governing how the no-fly zone is enforced. These guidelines define who is and isn’t allowed to fly in the airspace and they prescribe a process for determining if someone is in violation. They must also address the sticky situations: Are civilian airplanes allowed to fly? What if a civilian airplane wanders into the airspace with no flight plan? What if the adversary uses civilian airliners as screens? What if the adversary loads civilians into a plane, then violates the no-fly zone? What if the adversary shoots at friendly airplanes enforcing the no-fly zone? The answers must be clear to those enforcing the policy. If the guidelines are too broad, our aircrew will have to interpret them while dealing with murky situations in real time. If the guidelines are too complex, they may have difficulty keeping them straight when reacting to a tense situation.

Enforcing a no-fly zone usually requires a large amount of military forces, including aircraft, the operators who fly them, and support personnel to protect and maintain them. Unless the no-fly zone is relatively small, it will take multiple flying units operating different kinds of aircraft. This includes air-to-air fighters that can intercept adversary aircraft. Specialized aircraft are also required for suppressing or destroying the enemy air defenses that could shoot our aircraft down. We need to be able to know when someone violates the no-fly zone, which usually involves a combination of airborne and ground-based radar. Air refueling aircraft are required if our bases are far away from the no-fly zone, and they also extend patrolling time for our aircraft. We will need to monitor the adversary and look for dangerous actions (such as setting up surface-to-air missiles), which requires Intelligence, Surveillance & Reconnaissance (ISR) aircraft. Since we will want to retrieve our aircrew should they be stranded due to mechanical failure or being shot down, we need combat rescue assets in the area.

In addition to these forces, we need bases to house and protect them, infrastructure to support them, and secure logistics lines for the flow of supplies and people. In the past, the United States has typically used a combination of land bases and aircraft carriers to support no-fly zone operations. It’s good to have several bases from which to fly, as a runway problem or bad weather could close a base and halt the operation.

A coalition of participating states is almost always necessary for an effective no-fly zone. Putting together a coalition often involves arduous and time-consuming diplomacy, but the benefits of a coalition are vast. Coalition members provide bases and infrastructure, transit approval through their airspace, access to supply routes, and other support. A coalition can help to establish legitimacy for the overall mission, and coalition forces can share the burden of patrolling the skies, which reduces the cost for any one country.

How Much Will It Cost?

Enforcing a no-fly zone is expensive. In order to detect violators and take action against them, we must stand ready to respond at all hours, day or night. This means there is a lot of flying involved—perhaps more than in an attack campaign, where we can pick the times and places for our strikes (as in the campaign against the self-declared Islamic State over Iraq and Syria).

Even when airplanes aren’t flying, some aircraft and crews will need to be on alert status, ready to answer a provocation. This constant state of readiness is both expensive and taxing on our people. They must always be ready to fight, even as they endure long hours of boredom in the air—patrol sorties can extend eight to ten hours—and on the ground, far away from home.

There is also the opportunity cost associated with the no-fly zone. When large amounts of people and equipment are dedicated to enforcing a no-fly zone, they are not available elsewhere. This is an important consideration when you consider that many of the forces needed for the no-fly zones are both in high demand and limited in number. Air refueling, airborne radar, and ISR aircraft are good examples. Even fighter aircraft are limited in number today. For example, the U.S. Air Force had over 3,400 fighter aircraft in 1992, when the Iraq no-fly zones were put into place. Today, that number is less than 2000. The result is that implementing a no-fly zone in one part of the world will limit our options in other areas.

What Are the Risks?

There is a perception that a no-fly zone is less risky than other options. Despite this, decision makers should know they assume many types of risks when implementing a no-fly zone.

There will be significant risks to U.S. forces, especially if the adversary has equipment capable of shooting our aircraft down. Because of this, we may need to execute an operation to negate or destroy air defenses before we can start enforcing the no-fly zone. Given the increasing sophistication of air defenses around the world, this initial attack will not be a cakewalk. It will require a major offensive operation over many days of hundreds of high-risk sorties expending thousands of expensive munitions.

Once air superiority is established and the no fly zone in place, we must continue to stay alert to developments as the adversary learns and adapts to our tactics. They may try to establish ruses or deception plans to embarrass or attack us. In the 1990s, for example, Iraqi aircraft would violate the no-fly zone to lure us into the engagement envelope of a surface-to-air missile system they had recently repositioned.

If the adversary does shoot down one of our aircraft—or one goes down due to mechanical failure—there is a real possibility that our people will be captured and exploited. This will be difficult to resolve. It is quite possible that an unscrupulous adversary will torture or kill a prisoner in a gruesome, provocative way (as the Islamic State did when it burned alive a captured Jordanian pilot).

Perhaps the most significant risk is the risk of “mission creep.” It’s highly unlikely that the implementation of a no-fly zone will lead to the desired political outcome by itself. Inevitably, there will be calls to expand the mandate, perhaps to create a “no-drive zone” or to attack forces on the ground. For example, the no-fly zone over Libya in 2011 also involved significant strikes against Libya’s government forces in the effort to protect the opposition forces and civilians. Depending on the circumstances, expansion of the mission may be the right thing to do, but it leads to a final question…