FUNDAMENTALS OF DEFENSE Effective defensive actions are achieved by a blend of the principles discussed below. The degree to which each principle applies will vary with the mission and the situation. This is decided only after consideration of all the principles (fig. 4-2).

PROPER USE OF TERRAIN Maneuvers that are possible and dispositions that are essential are indelibly "written on the ground."

PROPER USE OF TERRAIN SECURITY MUTUAL SUPPORT DEFENSE IN DEPTH ALL-AROUND DEFENSE COORDINATED FIRE PLAN USE OF BARRIERS FLEXIBILITY MAXIMUM USE OF OFFENSIVE ACTION

DISPERSION

Figure 4-2.- Fundamentals of defense. Unfortunate indeed is the platoon commander or assistant company commander who is unable to read this "writing." The intelligent leader knows that terrain is a major ally and that it virtually decides the positioning of platoons and squads in the defense. The defender retains control of terrain features essential to observations, communications, and maneuver of reserves. Denying the enemy the use of terrain that might jeopardize the mission of the battalion is the major role of a defender.

To read the "writing on the ground," you can organize the analysis of weather and terrain primarily around the following set of military considerations (KOCOA):

Key terrain Observation and fields of fire Concealment and cover Obstacles to movement Avenues of approach

These considerations are discussed in the following paragraphs. While the discussions of these considerations focus on terrain, weather and terrain are inseparable. Terrain that offers good traffic conditions when dry may be impassable when wet. A hill that provides good observation on a clear day may not provide any visibility on a rainy day or at night.

Key Terrain Key terrain features must be considered in formulating defensive tactics. Their selection is based on the mission of the command. Tactical use of terrain often is directed at increasing the ability to apply combat power. Also considered is channeling the enemy into the beaten zone of organic weapons maintained by the battalion. The selection of key terrain varies with the following:

. Level of command l Type of unit . Mission of the unit

Seabees are normally a service support unit for the Marine Air-Ground Task Force (MAGTF). Service support units need roads over which to move supplies and secure areas in which to construct facilities. Aviation units need high terrain on which to set up radars and communication facilities and large, flat areas for airfields.

Observation and Fields of Fire Observation and fields of fire are so closely related that they are considered together. Fields of fire are based on observation because a target must be seen to bring effective fire upon it.

The observation considerations are as follows: . Weather conditions . Time of day . Vegetation . Surrounding terrain The highest terrain features are generally ideal for observation. However, during times of poor visibility, positions in low areas that the enemy must pass through may provide better observation.

The field of fire is the area a weapon or group of weapons may cover effectively with fire from a given position. When you are selecting a field of fire for a weapon, it is critical to know the capabilities of the weapon. Positioning an M60E3 machine gun with a 400-meter field of fire is poorly using the weapons capability considering the weapon has a maximum effective range 1,100 meters. The object is to engage the enemy as far out as possible.

Cover and Concealment Cover and concealment is used together to provide protection from the effects of fires and observation.

1. Cover is protection from the effects of fire. Examples of ideal cover are as follows:

Rocks Shell craters Ditches or quarries Buildings Caves Sunken roads River banks Walls Folds in the ground Highway fills Areas that provide cover from direct fire may or may not protect against the effects of indirect fire.

2. Concealment is the protection from observation or surveillance both air and ground. Some examples that provide ideal concealment are as follows

Woods Underbrush Snowdrifts Tall grass Cultivated vegetation Other features that deny observation

Terrain that provides concealment may or may not provide cover.

Obstacles to Movement Obstacles are anything, including a natural or artificial terrain feature, that stops, impedes, or diverts military movement. Entanglements are set up as obstacles along the FEBA or the defensive perimeter to channel the enemy into the beaten zone of various weapons held by the defending unit.

Avenues of Approach Avenues of approach are routes the enemy is likely to travel to reach its objective. When setting up the defense positions, the platoon commander visualizes all possible enemy avenues of approach into the area.

An evaluation of the avenues of approach, together with key terrain features, serves as the basis for positioning platoons or squads for planning fire support. The area selected for defense should afford good observation, fields of fire, and adequate cover and concealment to the defending forces. The defender applies the principles of camouflage and continuously improves camouflage throughout the defensive operation.