Although an opponent cannot be defeated with a defense alone, a good defense can cripple an attacker.
Defensive Operations are of critical importance to the attacker as well as the defender. An attacker cannot keep moving, eventually he must stop, sleep, eat, and basically recover. Whenever a unit stops it should immediately go into the defensive mode. This may not be obvious at the Battalion level but at the individual level it can be critical.
A Company, or Battalion (and up) will be a lot more methodical in the defense. Like any other type of military operation the defense is planned and executed.
In the Marines there is an acronym called SAFE-OPC that describes the priorities in the defense. S or Security is perhaps the most important. The enemy could attack at any time and should not be allowed to surprise the defender.
Enemy patrols will attempt to monitor the progress of the defense so they can note weaknesses. Enemy snipers will do their work and demoralize defenders, slowing down preparations.
Security patrols might be sent out along with LP/OP’s (Listening post/Observation Post).
A or Automatic weapons is also of critical importance. Machine guns should be placed where they can do the most damage. This means placing them where they can fire the farthest and achieve ‘grazing fire’. Machine guns should also be placed covering avenues of approach, where the enemy is most likely to come from.
F or Fields of Fire should be designated for everyone, including machine guns. A field of fire is basically an area of responsibility for a weapon or unit. FOF’s insure the entire front is covered by at least one weapon. FOF often interlock so more than one weapon is covering a certain area. Individuals, fighting positions (battle teams), fireteams, squads and up, are assigned sectors to protect. Additional weapons (like rocket launchers and anti-tank weapons) will be deployed to their most advantageous locations. Dead space (areas where the enemy can hide) are designated for grenadiers. Artillery and mortars are assigned pre planned targets and may fire a few rounds to insure accuracy.
Finally protective line should also be designated for machine guns. The FPL is a direction for the machine gun so that it fires almost directly across the units front. When the shit hits the fan, firing rapidly along the FPL can create a wall of lead that can stop an enemy attack from breaching the line.
Fields of Fire should also be cleared so the defenders can see more. This should be done as unobtrusively as possible so the enemy is not aware that he has stumbled into someone’s field of fire.
E for Entrenchment means when everyone is positioned they should dig themselves protection. Usually this means one person is providing security and the other is digging a fighting hole. The two will often rotate and the machine guns will always be manned and ready. Backup positions are also prepared.
O is for Overhead cover. Everyone should build shelter from artillery or air strikes.
P is for Protection. Minefields, booby traps, barbed wire and obstacles should be deployed. Minefields are best when someone can fire at somebody digging them up. Barbed wire has three purposes. Protective (keep the enemy outside of hand grenade range), Tactical (funnel the enemy into machine guns) and Supplemental (confuse the enemy as to what is protective and what is tactical). Barbed wire should also be booby trapped to discourage the enemy from trying to cut through it.
C is for Camouflage and Concealment. C should be practiced all the way through however and not just in the end. Machine guns should receive the most attention because of their importance in the defense. Machine guns usually end up in bunkers and regular riflemen end up in trenches.
SAFE-OPC is more than just a checklist. It should be repeated continuously. Defenders should always be improving the defenses and they should never stop.
Commanders should be located where they can best control the defensive network. A lot goes into the defense and a good defense can be nearly impenetrable. A great deal of planning usually goes on, especially in long term defenses. Food, water, ammunition supply points, command centers, mortar positions, artillery positions, sanitation should be considered, tank positions ect.
Doctrine usually calls for a two to one style defense. Two units are forward on the battle line, one is the reserve and manning the secondary line. A good commander always maintains patrols to his front and these patrols frequently come from the reserves although, when there are no reserves, they may come from the front line units.
Even when a unit stops for a little bit it should set out security, place automatic weapons, establish fields of fire and dig in. For brief stops a shallow little pit might be scraped out that is just big enough to lay down in. The longer a unit stays in position, the more elaborate its defenses get. Regardless of how long a unit is in the defense, it should continuously be improving and modifying the defenses. Usually troops will make little improvements of their own to make things more comfortable, like a shelf in their fighting hole to put magazines and grenades. Fire plan sketches should also be made. These are usually nothing more than sketches that detail things like what is pointing where, where dead space is, how dead space is covered, what the ranges are, ect. Fireplan sketches are usually kept for each hole. Fireteam leaders make a sketch of the fireteam’s area and submit them to the squad leader. Squad Leaders combine the fireteam sketches and provide the squad sketch to the platoon commander, ect. Sometimes the Battalion Commander will have a sketch that is a composite of all the individual positions.
Fireplan sketches also allow another unit to come in and take over the defensive positions with a minimum of trouble and confusion. I have seen positions in South Korea already prepared with fire plan sketches in case the North invades.
Rehearsals should also be conducted so everyone knows where the backup positions are and how to find them, day and night.
The actual placement of a unit depends on the defense used. Strong points can be used for a small unit to cover a larger area. Basically this means that the unit is divided into sub units and each one is assigned a position (like a hill top). From this position the strongpoints should be able to provide covering fire for each other. This means that if one strong point is attacked then the attackers will get shot to by the defenders they are attacking and the defenders of another strongpoint.
A standard defense is when a unit circles the top of a hill or ridge line. From that high point, they can see and shoot farther than someone down below. Also, the attackers must attack up hill and besides being very tiring, it can slow down an attack from sheer exhaustion. In Vietnam this was the most popular. Khe Sahn was an example of this.
A reverse slope defense is the sneakiest and relatively hard to dislodge. Because it is on the back of a hill the enemy has to go up and over the hill. When they reach the top of the hill, they make nice targets for gunners down below, especially at night.
This type of defense also is hard to get at for enemy artillery and mortars because of the angle. With spotters on the other slope, machine guns can be used to fire indirectly at approaching enemy troops. Basically, the machine guns are firing and the enemy can’t shoot back because they don’t have a target.