Defense in Depth

Defense in Depth (also known as "Delaying Action") is a type of Scenario where the players' Side attempts a slow retreat under constant fire. Players start in a prepared and/or fortified forward position, fighting an ever-increasing enemy force. As the enemy attack intensifies (usually whenever the enemy's Artillery manages to zero-in on the current player position), the players gradually Fall Back to a defensive position further back. Eventually, the players will Fall Back to an Alamo, which they either defend to the death or expect to be Extracted from.

Defense in Depth missions have become increasingly more common during the ArmA 3 era. The player Side is often Motorized, allowing the players to quickly fall back to the next fighting position. Close Air Support may be available, as well as various types of Weapon Teams. Although this has rarely been seen in videos, it appears that Command has the opportunity to set up each defensive position before the mission begins, installing fortifications and placing Resupply Crates at each position.

Early Defense in Depth missions were mostly Adversarial. In fact, most Adversarial missions during the early ArmA 3 era were Defense in Depth scenarios. Cooperative missions of this type grew ever more common over time, until ShackTac stopped playing any Adversarial missions altogether.

Concept
Defense in Depth is a military term that describes an operation intended to delay an enemy force's advance for as long as possible, while acknowledging an inevitable loss of territory in the process. It is also called a "Delaying Action" for this reason.

Defense in Depth is executed when time is more valuable than territory: If the enemy's advance is unstoppable, or if the territory held by the ground forces is less valuable than the chance to regroup elsewhere or carry out a stronger attack elsewhere, the defending force is ordered to gradually withdraw from their current defensive positions rather than try to hold them. Instead of a quick withdrawal, a Defense in Depth attempts to hold each position only so long as it is tenable. Once the enemy's attack poses a serious threat to the defenders, they are instructed to withdraw to the next position back.

Famous Defense in Depth actions in the real world are numerous, with the first being Hannibal's battle at Cannae, where his center performed a slow withdrawal to draw Roman legions forward, only to suddenly attack them from both flanks with fresh troops and annihilate a great number of them. The most famous recent example, and one of the most successful ever recorded, was the Battle of Kursk during World War II, in which the Soviet army performed a similar maneuver against the Germans, on a much larger scale. The Japanese also used Defense in Depth several times in order to substantially increase American casualties during the American island-hopping campaign in the Pacific Ocean.

The Setup
Before a Defense in Depth mission begins, the Commanders of the defending side have the opportunity to perform several actions on the map:


 * 1) Survey the area, determining which positions should be defended, and setting up "Fallback lines" - each marking a step in the defensive tactic. Units will generally withdraw from one Fallback Line to the previous one, unless instructed to skip one or more lines.
 * 2) Select an Alamo - the final position to be defended. This may or may not be defined by the Mission Designer.
 * 3) Prepare fortifications at strong defensible positions along each Fallback Line. While these may be set up by the Mission Designer beforehand, it's possible that the defending Commanders can place fortifications themselves, make adjustments, etc. This has not been confirmed. Fortifications may include Sandbags, Bunkers, Barbed Wire fences, and sometimes even stationary weapon emplacements.
 * 4) Place Resupply Crates at each Fallback Line, anticipating which materials will be needed by each unit as they fall back. Note that this does not necessarily preclude the presence of a Logistics unit which will bring additional Resupply Crates to the units as necessary. Again, it is possible that the Mission Designer already places these supplies on the map as part of the mission, rather than Commanders making that choice.

Right before mission-start, players will Teleport to the forward-most defensive line, and establish themselves along that line. The mission only goes Weapons Hot when the players are ready to begin their defense.

Force Composition
While a Defense in Depth can technically be performed by any type of Ground force, ShackTac missions of this type almost invariably use a Motorized Infantry force - with or without Mechanized support. Weapon Teams and Logistics units are also quite commonly added in, while Close Air Support is significantly less common.

As a Motorized force, the units have their own light vehicles - typically Trucks and Recon Cars - which they use to quickly fall back. The vehicles are generally kept in a safe spot just behind the defensive position, to be used only for the fall back maneuver. If any of the vehicles are armed, they may be used on the front line - at a risk of losing the vehicle and reducing the overall transport capacity. If any of the defenders are left without free seats in any vehicle, the withdrawal could be unnecessarily complicated, or require the entire force to fall back on foot.

Additional heavier vehicles (such as IFVs or Tanks) may be included as well, to protect the infantry against heavy enemy vehicles. However it is more common to see vehicle-mounted HAT teams (e.g. using a TOW HMMWV), which are both lighter and faster to move than an IFV or Tank, making them fit better with a Defense in Depth strategy.

A Logistics unit is almost always included in the ground force, bringing ammunition to the front lines and repairing any damaged transport vehicles. Keeping the vehicles in good condition is especially important, for the reasons stated above. Note that there may be Resupply Crates pre-placed at each fallback line, but Logistics may still need to run supplies to units forward of the fallback line or simply away from wherever the cache was placed.

Close Air Support is significantly rarer in Defense in Depth scenarios, possibly because it tends to tip the balance significantly in favor of the defenders. There may be other reasons why aircraft are rarely seen in these missions. Nevertheless, many Defense in Depth missions end with friendly aircraft joining the battle to turn the tide (or simply to save the defenders from destruction as they Extract).

Fighting and Falling Back
Players start a Defense in Depth mission already at their forward-most combat positions. Soon after Weapons Hot, the enemy begins assaulting the line with various ground forces.

The Players must attempt to hold the line for as long as is feasible without incurring serious casualties. Depending on the quality of the defensive positions, this can last for many dozens of minutes. Meanwhile, the enemy assault gradually becomes stronger over time, with more and more enemy units assaulting the players simultaneously. Eventually, however, one of three things will happen:


 * 1) The enemy assault becomes powerful enough to threaten a breach of the players' line, or begins to take casualties among the players.
 * 2) The players begin running out of ammunition and/or medical gear, and Logistics (if present) cannot resupply them all quickly enough.
 * 3) The enemy begins to shell player positions with accurate Artillery.

If any of these criteria are met, Command will issue an order to one or more units to begin retreating to the next Fallback line. This can be done in an orderly fashion (one unit at a time), or in a more urgent manner (any unit that can go, goes immediately), depending on circumstances.

Units typically mount their vehicles - if they have any remaining vehicles - and hightails it to the next Fallback line. When they reach the next line, they park their vehicles in a safe place and assume a defensive position in their assigned area. If possible, units will attempt to resupply from nearby Resupply Crates while waiting for the enemy to arrive.

Eventually, the enemy will reach the new fallback line, starting the process all over again.

Note that in some missions, withdrawal is not triggered by the strength of the enemy onslaught, instead being more contingent on the mission timer. In these missions, friendly forces have a timetable or deadline to follow, meaning that they must retreat all the way to the Alamo within a certain timeframe. Each withdrawal will be times accordingly, whether the enemy threatens to breach the defenses or not.

Alamo
The Alamo is the final fallback line in a Defense in Depth mission. Instead of an actual line to be held, it is one specific location where all friendly units will congregate for the final defensive effort.

The Alamo is typically a heavily-fortified or easy-to-defend position. It may be a large bunker complex, a construction site, a large compound, or any other similar group of structures that offers strong protection against enemy attack. It must be large enough to offer fighting positions for all arriving units, otherwise a Clusterfuck could form that puts the defenders at unnecessary risk.

If properly defended, the Alamo is exceptionally difficult to breach. Players in upper levels of the Alamo engage approaching enemies, while other players guard any possible approach against any enemy that manages to get through. If a sufficient number of players have survived long enough to reach the Alamo, they can potentially hold off a very large enemy force for quite some time. The idea is to make the enemy pay dearly for any attempt to assault the building.

On the other hand, the Alamo cannot hold the enemy off forever. Casualties will begin to mount, and friendlies will begin to run out of ammunition and supplies. Furthermore, the Alamo offers a large target for enemy Anti-Tank Launchers, Cannons and Artillery, which could ultimately breach the defenses and/or bring the Alamo itself down. If the players are not meant to Extract from the Alamo, and cannot manage to do so in time, they will eventually be killed defending it.

Extraction
Some Defense in Depth missions are designed as a no-win scenario, with the players expected to die defending the Alamo while taking out as many enemies as possible.

Other missions offer the players a potential for survival by being Extracted from the Alamo or from a safer location nearby.

Extraction during Defense in Depth missions can come in the form of Helicopters or Boats. Sometimes, a group of boats is already waiting for the players near the Alamo, though they are expected to attempt to defend the Alamo for as long as possible before retreating to the boats. At other times, the extraction vehicles will arrive some time after the Alamo was reached.

Helicopters may be able to land on the roof of the Alamo itself, though this can be very risky if the roof is within the firing line of enemies on the ground. Alternatively, players may organize a Landing Zone near the Alamo, retreating to that landing zone a short time before the helicopters arrive to pick them up.

If extraction is the ultimate goal of the mission, "success" may be measured by the number of friendlies who made it out alive, as well as the total amount of time they managed to continue fighting before the extraction.

Notable Examples
Defense in Depth has become increasingly common in recent years. Nevertheless, examples of this type of scenario are still somewhat rare. The list below should include all examples of those missions.


 *  by Dslyecxi: In this unique scenario, the players Paradrop into enemy territory to disrupt an enemy rally point, then draw the enemy towards friendly lines where they expect to link up with friendly Tanks. The player Side is an under-strength Motorized Infantry Platoon riding in air-dropped HMMWVs, accompanied by a .50 Cal HMMWV and a HAT Team. Since the action takes place in enemy territory, there are no prepared positions to fall back to; Instead the units withdraw through a series of small towns. Resupply is handled mostly by air-drops near each defensive position.