Enfilade and defilade
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Enfilade and defilade are military tactical concepts used to describe a military formation's exposure to enemy fire. In addition, enfilade fire is used to describe gunfire directed against an "enfiladed" formation or position. The words themselves come from French (enfiler: to skewer—défiler: here, to scroll).
[edit] Enfilade
A formation (or position) is "in enfilade" if enemy fire can be directed parallel to the long axis of the formation. For instance, a trench is enfiladed if the enemy can fire down the length of the trench. A column of marching troops are enfiladed if fired on from the front such that the gunfire travels the length of the column. A rank of advancing troops is enfiladed if fired on from the flank.
The benefit of enfilading the enemy is that, by firing along the long axis, it is relatively easy to hit a target (or several targets). Raising or lowering the aim of a weapon merely directs the fire to a different point along the axis of the position, though any rotation of the weapon is more likely to result in a miss. Enfilade fire takes advantage of the fact that aiming at a target is relatively easier than correctly estimating the range to avoid shooting too long or short. The less desirable alternative is to fire from an oblique angle which requires precise aim to hit a target. Raising or lowering the aim when firing obliquely results in the target being straddled, while rotation redirects fire towards an alternate target. Finally, shells that miss the target are more likely to hit a different enemy if firing down a line of troops rather than across their position.
[edit] Defilade
A unit or position is "in defilade" if it is protected from direct exposure to enemy fire. For an armoured fighting vehicle, defilade can be synonymous with hull-down.
Defilade is often used to refer to a position on the reverse slope of a hill or within a depression in level or rolling terrain. Defiladed positions on hilltops are advantageous because it allows a defender to take advantage of the height of the terrain without suffering the disadvantage of being silhouetted against the sky. However, because of the slope, "dead space" that cannot be engaged with direct fire will be created in front of the position. Ideally this dead space should be covered by the interlocking fields of fire of other nearby positions, and/or by pre-planned indirect fire such as mortars or artillery.
In the case of tanks or other armored fighting vehicles firing from a reverse slope, the tank will move forward only far enough that the gun when fully depressed can engage targets. This greatly reduces the size of a target that the vehicle presents, and because the hull is on a slope the effective slope of the front armor is also dramatically increased. Because of this, the degree to which a tank can depress its main gun is a significant advantage—the larger the angle of depression the less of a target the tank needs to present to enemy fire.
In the case of antitank weapons, and especially short-range man-portable antitank rockets, defiladed positions behind a hill have several important advantages. This is because the dead space created by the intervening crest of of the hill prevents an approaching tank from using the range of its direct fire weapons, and neither attacker nor defender will have a clear shot until the tank is within range of the defending antitank weapon. In such engagements the tank is usually at a further disadvantage because the defender will often be camouflaged while the attacking tank will be silhouetted against the sky, giving the defender an easier shot. In addition, if the tank fails to detect the defending antitank weapon while the tank is still defiladed, instead advancing until it has fully reached the crest of the hill, and may expose the relatively thinner armor of its lower hull to the defender. This is an important reason that it is imperative that tanks advance with infantry support.
Artificial entrenchments can provide defilade by allowing troops to seek shelter behind a raised berm that increases the effective height of the ground, within an excavation that allows the troops to shelter below the surface of the ground or a combination of the two. The same principles apply to fighting positions for artillery and armored fighting vehicles as well.
