The fundamental element of action is simply called a "Unit". There are 9 types of units; the types are separated into three categories according to the medium in which the unit moves. The categories and types are:
The fundamental actions that involve a unit are:
In addition, there is a pseudo-land-unit, the Anti-aircraft gun. Like the true units, AA-guns can be purchased, can destroy other units, and can move (in a limited fashion). Unlike the true units, AA-guns can't be destroyed, their ownership can change, and their movement is separate from the movement of true units.
"Axis & Allies" includes the "Industrial complex" among the list of units, but Concurrent_AA doesn't, because: they can't be destroyed, they don't have fixed ownership, they don't move, and they don't participate in combat. The only attribute they share with units is that they can be purchased.
Land units must move on land territories. They can be moved across sea zones on board a transport to a new land territory. Land units are the only units that effect a change of ownership of a land territory, and hence are the only vehicle for a nation to acquire new territory and increase its economic wealth.
Infantry units are the least expensive unit type (default 2 credits). They can move one territory per turn. Their attack strength of 1 is the lowest possible. Their defense strength of 2 is much better, making them better at defense. Nonetheless, they are essential to attack moves, since being the least expensive, they are the best choice to lose during combat. Of course, they must be advanced into battles in order to acquire new territory and effectively defend new acquisitions.
Armor units are the more aggressive land unit type. They cost 5 credits. They can move 2 territories per turn. Their attack strength is 3, effectively 50% accurate. They defend with 2, the same as an Infantry unit. While some Infantry units might not get orders to move on a given turn, it is generally best to order Armor units to move to get the most out of their aggressive attack strength. Because of its movement limit of 2, an Armor unit can capture an enemy territory and continue moving if that territory is unoccupied and uncontested by enemy units moving into it.
Sea units must move in sea zones. Some sea units interact with land units and land territories, however. All sea units can move two zones per turn.
Battleship units are rather expensive at 24 credits. Both their attack and defense strengths are 4. In addition, a battleship can make a single-shot bombardment against an island or coastal land territory if it accompanies an amphibious assault and wasn't otherwise involved in combat on the same turn.
Aircraft carrier units cost 18 credits. They attack with 1 but defend with 3. They have a special relationship with the unit type Fighter: up to two Fighter units owned by their same nation can land on an Aircraft carrier after the movement and combat phases, thereby extending the potential range of Fighter units, especially over sea zones.
Like Battleship units, Destroyer units are a general purpose combat sea unit. They are much less expensive at 8 credits. They attack and defend with 2. They are particularly suited to escorting Transport units.
Submarine units are superficially similar to Destroyer units: they cost 8 credits and they attack and defend with 2. Submarine units have several features and limitations not shared with the other sea units. Submarine units can't destroy air units, despite being susceptible to destruction by them. To counter this limitation, Submarine units, whether on the attack or in defense, can retreat from a battle in progress if the only enemy units present are air units. Submarine units also have a stealth mode. When moving in stealth mode, a submarine unit will not cause a conflict with any enemy sea or air units, potentially even passing by enemy units unnoticed. A submarine unit might still find itself in conflict at the end of its move if the final sea zone to which it moves also contains enemy units.
Transport units have no attack capability, although they can defend with 1. They have a very important function that makes them essential to winning the game: they can ferry land units of their same nation across sea zones to land territories which the land units might not otherwise reach. A coodinated set of orders, described here, is necessary to do this.
Air units may move freely through both sea zones and land territories. Air units' movement is different to both land and sea units in that their movement is not impeded by other units. Air units may fly over enemy territory or units without interacting with them at all. See the description of combat for details about air unit movement. Air units have longer movement range that non-air units, but with an important limitation: air units can't be ordered to use their entire movement allotment before combat. Some of their movement allotment must be kept in reserve in case they are involved in combat and are required to vacate the territory/zone in which the combat takes place. This movement reserve can be used in the post-combat phase to move the air unit to a safe land territory.
Fighter units cost 12 credits. They attack with 3 and defend with 4. They can move 4 territories/zones per turn. Because of the special relationship with Aircraft carrier units, a Fighter can be ordered to move 4 territories/zones before combat if the final position is a sea zone that contains an Aircraft carrier owned by the same nation, or an Aircraft carrier of the same nation is ordered to move to the same sea zone. Otherwise, a Fighter can be ordered to move at most 3 positions before combat.
Bomber units cost 15 credits. They attack with 4 and defend with 1. They can move a total of 6 territories/zones per turn (pre-combat plus post-combat). Bomber units have a special attack mode called "industrial raid". In this mode, a bomber unit is ordered to move to an enemy territory containing an industrial complex. If the bomber is not shot down by anti-aircraft fire on the way, it inflicts economic damage on that enemy at the rate of 3.5 credits per bomber (rounded down).
The Anti-Aircraft gun is superficially similar to the real units. They can be purchased for 5 credits. Only one AAgun can be in any given land territory. The owner of the territory is also the owner of the AAgun. An AAgun is never destroyed. They don't participate in regular battles; however, they shoot (with a strength of 1) at every enemy air unit that flies over the territory in which they reside. They can't be given orders, but they can be moved a single territory during the post-combat phase.