What’s wrong with “signature strikes”?

October 26, 2016
What’s wrong with “signature strikes”?
Scott Englund
Scott Englund Non-Resident Fellow, Counter-terrorism

The United States has increasingly relied on the use of Remotely Piloted Aircraft (RPAs) to conduct lethal strikes in its on-going counter-terrorism campaign.  RPAs (or “drones,” colloquially) are employed in a variety of missions, the majority of which are un-armed reconnaissance missions where highly sensitive sensors and real-time high-definition video are used to provide battlefield intelligence.  In fewer missions, drones carry air-to-ground missiles, and among these, an even smaller number deploy those munitions.  Lethal missions are divided into two classes: targeted, and “signature.”  A so-called “signature strike” is one where the identities of the individuals targeted are not known ahead of time, but their behavior—observed over several days—fits a certain pattern that leads the observers to believe that they are involved in a terrorist organization.  This could mean that they are seen with weapons, at a location known to be frequented by terrorists, or being observed to communicate with known terrorists.  While targeted killing (where the identity of the target is known, verified, and the individual is of sufficient importance to warrant lethal force) may be necessary in some cases, the “signature strike” tactic has some significant problems.

Signature strikes make it difficult for the United States to argue that it’s counter-terrorism operations strictly adhere to international law and common practice.  Signature strikes may violate two important principles in the law of armed conflict: 1.) the principle of discrimination: targeting military objectives, while protecting civilians and their property, and 2.) the principle of proportionality: using force that is proportional to the expected military advantage gained by an attack.   Apart from problematizing the application of these principles, the signature strike tactic suffers from a lack of transparency. Indeed, this lack of transparency makes it even more difficult to accurately assess the effectiveness and ethical or legal status of the tactic.

Some assumptions need to be made before I continue.  First, the United States believes itself to be at war, albeit against a non-state armed group, and has been at war since its Congress passed the Authorization for the Use of Military Force (AUMF, Public Law 107-40) September 14, 2001.  Second, International Humanitarian Law, even parts of that body of law that the United States has not ratified, applies to its on-going armed conflict.


The Principle of Discrimination

The principle of discrimination in the law of war is the long-held belief that non-military objects and those not participating in combat are excluded from legitimate attack, and should not be either subject to indiscriminate attack or subject to injury related to an attack on military objectives, within certain reasonable limitations.  Care must be taken to avoid causing damage to civilians and their property in the pursuit of military objectives.  Defining who is and who is not a combatant, and what is a legitimate military objective is therefore important.  International Humanitarian Law, as established by several historical documents such as The Hague and Geneva Conventions, in the practice of warring armies over the centuries, and in the domestic laws and military regulations of states involved in armed conflicts, sets out the “rules of war” and describes how to protect non-combatants from the inevitable devastation that accompanies war.

Article 52 of Protocol I Additional to the Geneva Convention defines a military object as: “those objects which by their nature, location, purpose or use make an effective contribution to military action and whose total or partial destruction, capture or neutralization, in the circumstances ruling at the time, offers a definite military advantage.”  The U.S. Army Field Manual on the Law of Land Warfare stipulates that any fortified or defended place is subject to aerial bombardment, these include:

“A place which is occupied by a combatant military force or through which such a force is passing…Factories producing munitions and military supplies, military camps, warehouses storing munitions and military supplies, ports and railroads being used for the transportation of military supplies, and other places devoted to the support of military operations or the accommodation of troops may also be attacked and bombarded even though they are not defended.”[1]

Paragraph 48 protects certain types of structures, “buildings dedicated to religion, art, science, or charitable purposes, historic monuments, hospitals…provided they are not being used at the time for military purposes.”[2]

Defining “combatants” is somewhat more difficult.  A combatant is traditionally a member of the armed forces engaged in armed conflict between two states.  Combatants wear uniforms, are organized into recognizable formations, carry their weapons openly and respond to and give orders using a discernable chain of command.  In some cases, as in Additional Protocol II to the Geneva Convention, combatant can also refer to “dissident armed forces and other organized armed groups.”  Combatants have the right to directly engage in hostilities and are granted certain immunities in exchange for becoming legitimate targets themselves.  For example, if captured, combatants can expect to be treated as a prisoner of war, not be tried and convicted of any domestic crimes allegedly perpetrated during hostilities, and can expect to be repatriated at the conclusion of hostilities.  Civilians who engage in hostilities are sometimes referred to as “illegal” or “unprivileged” combatants who may not be entitled to prisoner of war status, though are not immune from attack.  The specific nature and duration of a civilian’s engagement in hostilities, which would deprive them of their protected status, is open to interpretation.  According to some, and problematic to the signature strike tactic, one’s potential to engage in hostilities does not render that person a combatant.  The International Committee of the Red Cross has concluded that,

“While in some countries, entire segments of the population between certain ages may be drafted into the armed forces in the event of armed conflict, only those persons who are actually drafted, i.e., who are actually incorporated into the armed forces, can be considered combatants. Potential mobilization does not render the person concerned a combatant liable to attack.”

To that end, the U.S. Government’s position is that while military-aged males may be combatants, “it is not the case that all military-aged males in the vicinity of a target are deemed to be combatants.”

Having defined what constitutes a legitimate target of military action, what requirements exist to put the principle of discrimination into practical effect?  Article 57 of Protocol I Additional to the Geneva Convention states:

“1. In the conduct of military operations, constant care shall be taken to spare the civilian population, civilians and civilian objects.

2. With respect to attacks, the following precautions shall be taken:

                  (a) those who plan or decide upon an attack shall:

(i) do everything feasible to verify that the objectives to be attacked are neither civilians nor civilian objects and are not subject to special protection but are military objectives within the meaning of paragraph 2 of Article 52 and that it is not prohibited by the provisions of this Protocol to attack them;

(ii) take all feasible precautions in the choice of means and methods of attack with a view to avoiding, and in any event to minimizing, incidental loss of civilian life, injury to civilians and damage to civilian objects”

It seems unlikely that signature strikes, in which the precise identities of the individuals targeted is not known, can be said to meet the test that commanders “do everything feasible” to be sure that they are indeed attacking military objectives.  That uncertainty would also make it difficult to avoid or at least minimize the chance of killing or wounding civilians or damaging their property.


Principle of proportionality

Proportionality means that the force used against a military objective must be proportional to the military advantage that could be gained by attacking it.  In trying to achieve a military advantage, the least amount of lethal force should be used in order to minimize the risk to civilians and their property and to avoid causing excessive suffering even among combatants.  The U.S. Army Field Manual on the Law of Land Warfare orders:

“Those who plan or decide upon an attack, therefore, must take all reasonable steps to ensure not only that the objectives are identified as military objectives or defended places within the meaning of the preceding paragraph but also that these objectives may be attacked without probable losses in lives and damage to property disproportionate to the military advantage anticipated.”[3]

Commanders must be sure that the risk of harming civilians and civilian objects is not greater than the military advantage gained.

The U.S. Counter-insurgency (COIN) field manual, notes that, “sometimes doing nothing is the best reaction,” and “some of the best weapons for counterinsurgents do not shoot.”[4]  Proportionality has a different character in irregular warfare; the COIN manual notes that, rather than a clear calculation of military benefit versus the risk of collateral damage,

“in a COIN environment, the number of civilian lives lost and property destroyed needs to be measured against how much harm the targeted insurgent could do if allowed to escape. If the target in question is relatively inconsequential, then proportionality requires combatants to forego severe action, or seek noncombative means of engagement.”[5]

Applying this observation to signature strikes in the U.S. counter-terror mission is problematic: if the identity of the individuals targeted is not known, but is based instead on a pattern of behavior, then it may be very difficult to make the appropriate calculation.  Determining the military advantage of attacking an individual, or group involved in an activity that appears hostile may not be possible from aerial surveillance.  Signature strikes appear to risk causing excessive destruction that is not proportional to the military advantage gained, and may in fact be counter-productive.  As noted for the COIN environment, which can be applied here, “An operation that kills five insurgents is counterproductive if collateral damage leads to the recruitment of fifty more insurgents.”[6]


Lack of transparency

One of the most consistent complaints about the United States’ drone campaign is that it is difficult to reliably assess the program without more detailed information from the U.S. government.  Additional information on how the United States creates target lists, the measures it takes to minimize collateral damage, and a summary of casualties is frequently called for.  These were among the Stimson Center’s recommendations found after its year-long study of the program.  Secrecy is necessary in carrying out military operations.  However, greater transparency will serve to fill an information vacuum currently occupied by terror-group propagandists who adroitly use collateral damage caused by drones to recruit, and may help bolster support for drone operations among allies of the United States.  Of course, transparency would also provide important information to legitimate targets, which they could use to avoid becoming a casualty.  This is a natural limitation of a tactic that seeks to minimize risk to U.S. personnel while at the same time tries to curtail collateral damage that inevitably comes from air strikes.  The unconventional nature of the conflict in which the United States finds itself will require creative solutions to the problem of transparency.

The White House tried to dispel charges that the U.S. government drone program is excessively secretive by issuing two statements in early July 2016.  The first was an Executive Order that instructs the executive branch to, “continue to take certain measures in present and future operations” to comply with international law and minimize the risk to civilians, and also orders the Director of National Intelligence (DNI) to publish an annual report on strikes undertaken “Against Terrorist Targets Outside Areas of Active Hostilities.”  The DNI, in accordance with the Executive Order released a summary, (barely more than two pages) which tabulated the number of strikes, and the number of dead combatants and non-combatants.  It also laid out possible reasons for the discrepancy between U.S. government casualty estimates and those of non-governmental organizations.  The DNI assessed that generally its estimates may be more accurate because it can draw on a wide range of sources, to include sensitive intelligence sources, such as human assets in the field, signals and geospatial intelligence.  “This combination of sources is unique and can provide insights that are likely unavailable to non-governmental organizations.”  This hardly does much work to allay concerns that the U.S. government is not being as forthright as is might need to be.



Not all drone strikes are created equal.  Some are necessary to find and kill dangerous people committed to doing greater harm to innocent people.  The foregoing has not been an appeal to pacifism, nor meant to argue that drones are an aberrant form of warfare that has strategically shifted the nature of combat.  The United States’ fifteen-year-old counter-terror campaign, being essentially limitless in time and geography, has been criticized in a number of ways.  Under present leadership, the U.S. government has dramatically escalated its use of unmanned aircraft to execute its counter-terror mission abroad, outside “areas of active hostilities.”  This has been controversial.  Signature strikes, where the identities of those targeted and killed are unknown, are particularly problematic.  It is difficult to establish that such strikes can meet all the requirements of discrimination and proportionality.  An individual’s status as a combatant is dependent upon some facts that one may not be able to determine from aerial observation alone.  Furthermore, proximity to a legitimate target should not be risky on its own.  Patterns of behavior that are determined to have hostile intent may not provide enough information to calculate the military advantage that might be gained by an attack.  Without a good estimate of the value of a military objective, planners cannot ensure that incidental civilian casualties or damage to their property is proportional to the military advantage that might be gained.  These problems might be mitigated though greater transparency on behalf of the U.S. government officials responsible for planning and conducting its drone operations.  To the extent that these problems cannot be overcome, the program should be modified.  Like the crossbow and harquebus centuries ago, new means of waging war have run ahead of our ability to clearly evaluate the ethics and legal status of those means; it is reasonable to expect that new weapons systems be subject to thorough ethical review.

[1] FM 27-10, Paragraph 40 c

[2] FM 27-10, Paragraph 45

[3] FM 27-10, Paragraph 41, p. iv

[4] FM 3-24, Paragraphs 1-152, 1-153, p. 1-27

[5] FM 3-24, Paragraph 7-32, p. 7-6

[6] FM 3-24, Paragraph 1-141, p. 1-25