The Massive Ordnance Air Blast Weapon (MOAB, known informally as the “Mother of All Bombs” and formally as the Guided Bomb Unit, or GBU, 43/B) has attracted a great deal of attention since it was dropped on ISIS fighters in eastern Afghanistan on 13 April.  Unfortunately, discussion of the attack has been hobbled by a lack of understanding as to the nature of the weapon and the uses for which it was designed, as well as the tendency to discuss it by reference to nuclear weapons.  In this article, we hope to add some needed granularity to the analysis of the attack by describing the MOAB and its purpose.  We conclude by highlighting the international humanitarian law issues implicated by the attack.

Developed in a relatively short time during 2003 with a view to use in Iraq, the MOAB was never employed in that conflict.  The intent was to use it against large formations of troops or hardened above-ground bunkers.  It was also intended for “psychological operations” targeting enemy morale, both by virtue of the size and extent of the resulting blast and the fact that it creates a large mushroom cloud resembling that of a nuclear detonation.  Thus, it was seen as a particularly useful weapon for “shock and awe” type tactics.

The MOAB is huge by conventional bomb standards. Weighing in at approximately 11 tons, it contains 18,700 lbs. of H-6 explosive, which was originally developed for underwater explosions due to its low sensitivity to shock and stable storage characteristics.  This is the largest quantity of explosive in any non-nuclear weapon in the US inventory (although there are larger weapons by weight, they contain less explosive due to having heavier casings designed to penetrate targets).  By way of comparison, the frequently-used Joint Direct Attack Munition (JDAM) comes in at launch weights of between roughly 500 lbs. and 2000 lbs.

Although the military has recounted “kicking it out of the back door” of the MC-130 aircraft, which suggests a “barrel-bomb” approach to employment, the MOAB actually is a guided munition. After leaving the aircraft, a parachute attached to the MOAB deploys.  The weapon subsequently detaches and is guided to a pre-determined target by GPS (satellite) and onboard avionics.  Thus, it differs from its predecessor, the obsolete BLU-82 “Daisy Cutter” bombs developed for use in Vietnam.  Daisy cutters were unguided “dumb” bombs that created large blasts areas in order to clear sites that could subsequently be used for helicopter landing zones. They saw some limited use in both Iraq and Afghanistan, including against cave complexes, until 2008.

The MOAB must be distinguished from the class of volumetric weapons, such as thermobaric weapons and fuel-air explosive weapons that have been used against fighters within cave complexes in Afghanistan.  Those use various means to create a cloud of burning particles with a wide blast radius and intense fireball.  When employed in a confined area, such as caves, thermobaric weapons can create a powerful vacuum as a secondary effect that adds to their lethality.

The MOAB is a conventional explosive weapon.  It results in an initial fireball from the explosion and a subsequent pressure wave caused by the creation of large quantities of gases at high temperatures. The MOAB is an air burst weapon, that is, it detonates above the ground.  This allows its destructive energy to dissipate over the widest possible surface area rather than being absorbed by the ground impact or reflected upwards. It can be used both to demolish surface targets and as an anti-personnel weapon. The effects are similar to any standard high explosive weapon, but the size of the blast radius (reported to be approximately 1 mile) is what marks the MOAB apart from smaller munitions.

While the fireball causes localized devastation, its lethality and destructiveness is primarily the result of the dissipation of the pressure wave.  As with any explosion, the wave creates blast injuries when it contacts body surfaces; gas filled bodily structures (lungs, ears, gastro-intestinal tract) are likely to be the most affected.  Deployed against a cave system, the pressure wave can injure or kill those within the caves, as well as cause tunnel collapse.  It is important to understand that the MOAB injures and kills in a fashion not unlike that of most other bombs.  Of note is its thin casing, which ensures a large blast radius, while reducing the chances of fragmentation injuries.

In the case of the 13 April attack, the effects of the weapon are apparent in video footage released by the Department of Defense.  It shows a target that is located within a narrow valley in a mountainous area of eastern Afghanistan.  The initial blast is confined by the surrounding hills before a distinctive mushroom cloud is seen forming. Local and U.S. reporting currently suggests that 36 ISIS fighters were killed with no civilian casualties and that the cave facilities were destroyed.

The MOAB can be viewed as a weapon with value at the tactical, operational, and strategic levels of warfare.  It is tactical in the sense that the weapon is especially useful against certain targets, such as caves and tunnel systems. General John Nicholson, Commander of U.S. Forces Afghanistan, noted that the MOAB strike served tactical purposes: “As ISIS-K’s losses have mounted, they are using [improvised bombs], bunkers and tunnels to thicken their defense…. This is the right munition to reduce these obstacles and maintain the momentum of our offensive against ISIS-K.” He added, “It was the right time to use it tactically against the right target on the battlefield,”

Beyond tactical use, the MOAB could serve to force the enemy to discard particular tactics to achieve operational level of war objectives. And at the strategic level, it is useful in signaling resolve and other strategic messaging. However, in light of its cost ($16 million each) and size, it is unlikely to be used with any frequency.

Despite the attention it has drawn, the MOAB presents no unique issues or challenges for international humanitarian law (IHL).   As a guided weapon, it does not run afoul of the prohibition on weapons that are by nature incapable of being directed at lawful military objectives.  On the contrary, the fact that it is a precision munition using GPS for guidance cuts the other way.  Nor is the MOAB a weapon that causes superfluous injury or unnecessary suffering, for its effects on combatants are basically the same as those of other blast weapons that rely upon the creation of a pressure wave to injure or kill.  It appears clear that the MOAB is not a weapon that is unlawful per se.

As with any weapon, however, its use in particular situations may be unlawful.  Of note in this regard are the rule of proportionality and the requirement to take precautions in attack.  But these IHL rules are always context-dependent. Consider the rule of proportionality, which prohibits “an attack which may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated.” Using a MOAB in a populated urban area, for instance, would generally violate the rule except in circumstances where the military advantage sought is enormous.  However, as this attack demonstrated, its use in remote areas where civilians and civilian objects are absent raises no proportionality concerns.  Proportionality analysis is never concerned with the degree or reach of the destructive force in the abstract. Rather, such calculations are always conducted with respect to the attendant circumstances.

The obligation to take precautions in attack to minimize harm to civilians and civilian objects takes numerous forms. It requires that an attacker verify the target, consider the issue of proportionality throughout the execution of the operation, select the target that achieves the effects sought with the least harm to civilians, and, when feasible in the circumstances, warn the civilian population of an attack that may affect it. None of these requirements present MOAB-unique concerns; again, compliance with them is context-specific.

In fulfilling with the precautions in attack requirements, an attacker also must consider the methods (tactics) and means (weapons) of warfare at its disposal to accomplish its objective, and select those that spare the civilian population to the extent feasible.  Considering its destructive force and range, employing a MOAB would be unlawful if the attacker could use one or more less destructive weapons, such as the JDAM, in order to limit the harm to civilians. This would likely be so if numerous civilians were present in the target area.  In this case, it would appear that the area was devoid of civilians, with their having fled the ISIS fighters or having been evacuated, paving the way for the use of the weapon. Note that the precautions requirement with respect to weapons choice does not apply with respect to harm to combatants or other fighters; any weapon may be used to engage the enemy so long as it does not cause superfluous injury or unnecessary suffering, or violate a specific prohibition on that weapon.

Finally, IHL forbids the use, or threat of use, of any weapon or tactic when the primary purpose of the operation is to terrorize the civilian population.  This is so even if the operation would otherwise comply with all other IHL requirements.  The MOAB could obviously be used in this fashion, as its immediate effects are stunning.  However, violation of the prohibition requires an intent to terrorize.  Absent such an intent, and without prejudice to other requirements of IHL, a decision to resort to the MOAB would be lawful. Relatedly, IHL does not currently consider unintended or incidental mental harm to civilians as part of proportionality analysis, though some might wish for the law to move in that direction.

It must be cautioned that this prohibition applies only to those operations designed to terrorize the civilian population. There is no equivalent prohibition on employing a weapon or engaging in tactics in order to terrorize enemy forces. Therefore, the fact that the MOAB was designed in part as a “shock and awe” effect weapon and may effectively demoralize, and even terrify, the enemy has no bearing on the legality of its use.

In sum, the well-publicized deployment of the MOAB has excited considerable interest and discussion, but its use in these circumstances is uncontroversial from an international law perspective taking into account the basic design features of the weapon, the nature of the target that it was employed against, and the apparent absence of civilians in the target area.

The views expressed are the authors’ own and do not necessarily represent those of the UK Ministry of Defence or the UK government, the US government, the US Department of Defense, or the Naval War College.

Image and video: DoD