Knock-on-the-Roof: The U.S. Air Force’s New Tactic


On April 26, 2016, U.S. Air Force Maj. Gen. Peter E. Gersten, the deputy commander for operations and intelligence (O&I) of Operation Inherent Resolve—America’s war against the Islamic State—revealed in a press conference a new tactic: “knock-on-the-roof.” According to Gersten, this tactic aims to “mitigate the loss of civilian life and minimize collateral damage.” On one hand, this tactic could symbolize the rise of a new norm for liberal democracies. On the other, this tactic could easily become futile in achieving its goals if treated just as a warning shot.

The Air Force utilized this tactic in an airstrike on Islamic State cash storage facilities on April 5 in Mosul. The targeted facility was the house of an Islamic State finance amir. Gersten claimed intelligence had shown a woman and her children regularly within the targeted facility. As a consequence, the Air Force needed to ensure these civilians were kept out of harm’s way. The selected solution was, in Gersten’s words,

to put a Hellfire [air-to-surface missile] on top of the building and air burst it so it wouldn’t destroy the building, simply knock on the roof to ensure that she and the children were out of the building. And then we proceeded with our operations.

Gersten added “they are using the civilian force as human shields, and we will fight and do everything possible we can to keep those civilian casualties to an absolute, absolute minimum.”

Unfortunately, despite the caution, the woman ran back into building seconds before the final impact and probably was killed by the airstrike.

The “knock-on-the-roof” tactic is derived from one of the main international humanitarian law (IHL) rules governing hostilities—precautions. It affirms that “effective advance warning shall be given of attacks which may affect the civilian population, unless circumstances do not permit.” The International Committee of the Red Cross (ICRC) asserts that this rule mandates an obligation to “choose means and methods of attack that avoid, or at least keep to a minimum, the incidental harm to civilians and civilian property.” Finally, the U.S. Department of Defense’s Law of War Manual, gives a more lenient definition stating that, “Certain affirmative duties to take feasible precautions to reduce the risk of harm to the civilian population and other protected persons and objects.”

When asked about the origins of this new U.S. Air Force tactic, Gersten answered that the Air Force adopted the tactic, technique, and procedure from Israel. A “knock-on-the-roof” is a long-standing Israeli Air Force (IAF) procedure that dates back to the end of 2008. The IAF procedure was developed as an improvised solution to an operational and moral paradox faced by joint operations centers (JOC) during the early days of the IDF’s 2008-2009 Operation Cast Lead in the Gaza Strip.

The JOC, comprised of representatives of the IAF, IDF’s Southern Command, the Military Intelligence Directorate, and the Israel Security Agency (Shin-Bet), was planning an airstrike on a residential house that was being used as a weapon and rocket storage facility. Under the precautions obligation of IHL, the center initiated a recorded phone call to the targeted house. The Arabic language message informed the person on the line that the IDF was aware of weapons stored in the home and was preparing to bomb the house. The residents had 30 minutes to evacuate.

Instead of leaving, the people inside went to the roof of the building for the purpose of deterring the IAF attack by using themselves as human shields. When the joint operations center saw this, they decided to deploy a second precaution – a small caliber air-to-surface munition targeted to a distant area of the roof. The goal was to make clear to civilians that the IAF would not hesitate to attack the building. Soon after the explosion of the small munition, all of the civilians fled unharmed and the IAF destroyed the building and its stashed arsenal.

Since then, the IAF perfected its “knock-on-the-roof” procedure, utilizing it frequently, with the most recent example during its 2014 confrontation with Hamas in the Gaza Strip (Operation Protective Edge).

It is important to understand that the precautions rule, and a “knock-on-the-roof” as one of its practices, are not simply a “warning shot” deterrence mechanism. As Frank Hoffman, a senior research fellow at the National Defense University and War on the Rocks contributing editor, explained to me via email in reference to the knock-on-the-roof tactic:

It’s more than a warning shot, it’s an evolution in praxis that seeks to balance the need to sustain strategic / international legitimacy while fighting an asymmetric opponent only too willing to shield his command and control, arms caches and criminal activity behind the populace.  Israel no doubt finds it ironic that its efforts to preclude senseless casualties are met with derision while Hamas’ callous disregard for life is rarely noted.

Contemporary conflicts have shown that wars can no longer be won by solely overpowering adversaries. Over the years, weak actors have adapted their strategies and tactics against the overwhelming power of their peers. They have done so not only via insurgency, guerrilla warfare, and terrorism, but also via the use of domestic or international law as a means to gain legitimacy or to de-legitimize the adversary (lawfare); and by exploiting the media to achieve political goals (imagefare). As such, the legitimate use of military force could be put in jeopardy and even restrained if it failed to follow the laws of armed conflict sufficiently.

In response, the tools employed by states  have evolved as well, to include new counterinsurgency and counterterrorism military tactics, legal support to military operations, and strategic communication. As a part of this legitimization campaign (legitimization-fare), the Israeli case can teach us important tactical and strategic lessons.

On the tactical level, a “knock-on-the-roof” enables military commanders to minimize harm to civilians, even if they are in the direct line of fire. Yet, similar to the American case mentioned above, in several attacks, Palestinian civilians that vacated a targeted building after a “roof-knock” procedure, returned to the premises while the airstrike was already taking place and could no longer be called off. Second, when targeted buildings are stocked with weapons and explosives, secondary explosions can harm unsuspecting civilians in the surrounding neighborhood who did not receive the same warning.

Last, and most important, the United Nations Independent Commission of Inquiry on the 2014 Gaza Conflict, noted that, “In some cases, it appears that concerned persons did not understand that their house had been the subject of a ‘roof-knock,’” especially when the tactic was used in the vicinity of other attacks in progress. Those tragic incidents resulted in the deaths of innocent Palestinian civilians.

Regarding the effectiveness of the tactic, the IDF sees the procedure as a final precautionary measure that complements other warning methods such as telephone calls, text messages, and leaflets. As such, Israel’s Ministry of Foreign Affairs explained, “While ‘roof knockings,’ like other kinetic means, may be imperfect, IDF assessments show that the employment of ‘roof knocking’ was highly effective, preventing many civilian injuries and deaths during the 2014 Gaza Conflict.” The Israeli Ministry of Foreign Affairs concluded that the IDF used this progressive precaution that went beyond the requirements of international law in certain instances where other warnings were unheeded or unfeasible.

On the other hand, the U.N. commission concluded that, “‘Roof knocks’ cannot be considered an effective warning given the confusion they often cause to building residents and the short time allowed to evacuate before the actual strike.”

Furthermore, in the contemporary character of conflict, tactics could have tremendous strategic results, and even policy consequences. Related to precautions as a lawfare practice, some international law scholars see the IDF procedures as a legal drawback. Michael Schmitt, the director of the Stockton Center for the Study for International Law at the U.S. Naval War College, concluded that, “The IDF’s warnings certainly go beyond what the law requires, but they also sometimes go beyond what would be operational good sense elsewhere.” Schmitt added that, “People are going to start thinking that the United States and other Western democracies should follow the same examples in different types of conflict. That’s a real risk.” As seen by the U.S. Air Force’s adoption of the “knock-on-the-roof” tactic, this statement has proven accurate.

While some may say a “knock-on-the-roof” contradicts the essence of feasibility of the precautions rule, it could become in the near future an international norm among liberal-democracies, thereby transformed into a binding customary international law.

Despite all this, perhaps the most important lesson to be taught from the Israeli case is the understanding that extreme precautions are not truly legal, but rather utilitarian. Restraining the use of military force does not necessarily mean maintaining the legitimization to use it.


In 2009, and again in the 2014 Israel-Hamas confrontations, the fight over international legitimization, became increasingly important in a social networked and globalized media environment. The “knock-on-the-roof” procedure could be a useful tactic in the arsenal of the fight over international legitimization and conducting legal efforts during conflict. Yet, it is most important not to see it as a bullet-proof solution to the operational and moral paradoxes found in warfare today.


Elad Popovich is an Israel Institute’s Research Fellow in the Arnold A. Saltzman Institute of War and Peace Studies at Columbia University’s School for International and Public Affairs (SIPA).