As previously stated, passive air defense is all measures, other than active air defense, taken to minimize the effectiveness of hostile aircraft. While both insurgent and professional organizations employ passive air defense tactics, it may be the insurgent’s only means of countering aerial threats. Passive air defense tactics, especially when employed by insurgent forces, may play more than just a role in determining a force’s survivability. Whether or not to engage an aircraft can be affected by several factors, such as the presence of a civilian populace or the goal of waiting for enemy forces to enter a compromising position. As stated in captured Viet Cong documentation, forces were not permitted to engage aircraft flying over densely populated areas to prevent retaliation, which civilians may become victim to (3). By attempting to prevent civilians from being targeted by enemy attacks, the populace is far more likely to continue cooperating. In terms of offensive benefits, waiting for a helicopter to land and deploy troops would place the enemy in a far more compromising position than if active air defense measures were employed as soon as the aircraft is detected in the air space.  

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Before discussing the historical uses of passive air defense in combat, individual measures and their subcategories must be defined. Passive air defense techniques can be divided into two main categories: attack avoidance and damage mitigation. These categories determine how a force will react when presented with an aerial threat and may be favored over the other depending on factors such as the size of a force, type of equipment, mission, or terrain features.

Attack Avoidance

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Attack avoidance refers to measures taken to minimize visible signatures that hinder an aircraft’s ability to locate a target. The three main measures include concealment, discipline, and construction. Concealment is the practice of selecting the most advantageous position to hide a man, an object, or an activity. Depending on the environment, these positions can be buildings, heavily wooded areas with a dense tree canopy, locations covered with thick foliage, or underground structures such as caves.

Discipline covers a broad spectrum of habits that strictly maintain proper concealment on individual and group levels. The disciplined maintenance of concealed positions limits oddities in the environment that may tip off a threat to the group’s position. These oddities can be as simple as an abundance of concentrated waste, such as used ration packs or vehicle tracks ending abruptly in a location that does not make sense for a vehicle to be under normal circumstances. To prevent revealing a position due to these oddities, a group may blend waste with debris that is already present or bury it. If vehicles have been used at the position, dusting away left behind tracks, following natural lines in the environment, or leading tracks away from the position will help prevent aircraft from quickly determining where the vehicle has stopped.  

There are two aspects to construction that, along with the previously mentioned measures, help reduce the visible signature of a location. Blending is the first aspect and the most likely to be used by an under-equipped insurgent group. Blending is the use of natural materials to blend with the surrounding terrain. Objects such as local fauna or camouflage with similar color tones to the environment may be applied to equipment and uniforms to help blend silhouettes with the surroundings. Disguising, the second aspect of construction is usually only implemented by larger forces and professional militaries. Disguising refers to the use of simulated vehicles or buildings meant to draw enemy fire away from actual positions. Equipment such as decoy inflatable tanks or empty tents are a couple of examples that may be used. 

Damage Mitigation

Damage mitigation refers to measures taken either during or prior to an attack meant to limit any damages an aircraft can inflict if a target is located. Damage mitigation measures are ideally implemented as a last line of defense if attack avoidance measures have not been successful in hiding a position from an aerial threat. Both measures can be taken prior to and after an air attack has commenced. 

Referred to as protective construction, this measure uses natural or manmade cover to shield a unit from incoming fire. Protective construction may be established during the construction of a position, such as a bunker, or it may be sought out during the attack, such as a natural defilade. The use of smoke to hinder an aircraft’s visual of a unit also falls under protective construction. 

Arguably, the more important measure is dispersion. Dispersion is the act of creating space between troops/equipment in order to limit potential damage from a single attack. Not only does practicing dispersion ensure that a unit will not be completely eliminated after an attack, but it also creates smaller targets, hindering target acquisition capabilities of an aircraft. (4)