An amphibious assault is an attack by units borded on transport ships on a coastal province. To be able to start an amphibious assault, the coastal province attacked needs to have a beach suitable for a landing party -- this can be checked by looking at the terrain map in the game. A province will have a small beach line if this is the case. Consult the image to the right. Tokyo has a beach, as indicated by the beach symbol on the terrain map. Additionally, if you click on the province a small beach symbol will be shown in the overview.
Amphibious Assault Efficiency
Often, a newer player will throw lots of units into the attack, including tanks and HQs. Then, they wonder why the attack fails miserably. An amphibious landing requires much preparation and coordination. This is why, in-game, an additional amphibious landing stacking limit exists. For Doomsday, the limit is 3 divisions, for Armageddon, the limit depends on your naval doctrines and can be between 3 and 8 (for the Base Strike 1952 tech). Every unit over this limit will decrease the combat effectiveness of all units by 10%, cumulatively. For example, let's say the command limit is 3. If you attack with 3 divisions, all attack with 100% efficiency. If you attack with 4, all four will attack at 90% efficiency. If you should chose to attack with 12 divisions, all units will have a penalty of (12-3)x10 = 90 percent!
This doesn't mean that one should never attack with more divisions than the limit. In fact, every additional division will add Org and draw fire from the other divisions. The marginal benefits of an additional unit's Org will have to be compared to the marginal loss of combat efficiency (10%, for each division over the stacking limit). The following table will demonstrate this for a stacking limit of 3.
Units Eff/unit Total Eff 1 100 100 2 100 200 3 100 300 4 90 360 5 80 400 6 70 420* 7 60 420* 8 50 400 9 40 360
Committing 6 or 7 divisions to the attack will yield the highest amount of total combat effectiveness deployed.
Best units for a landing
Obviously, marines are the best divisions for an amphibious landing. They get the lowest combat penalty in an attack. Second best are mountaineers. These are also very well suited for an amphibious landing and can clear a beach that is not so heavily defended and are more versatile as well as cheaper to build. Paratroopers also have less difficulty participating in a landing. But consider, if you have paratroopers, these would be better used to stage a simultaneous drop-off over the beachhead for extra efficiency. Plain vanilla leg infantry will also be capable of an amphibious invasion. Do not use Mechs, Mots, Arms or HQs, as these divisions have excessive amphibious landing penalties.
The amphibious landing penalties are reduced for each Marines tech researched. Even if you never plan to build marines and want to use Mnt or pure Inf in your landings, you should research the Marines techs, as the increased amphibious assault efficiency will make landings easier for all unit types.
As you would want to have as much firepower as possible during the attack, all units should be brigaded if you expect stiff resistance. Artillery for marines is a good idea, leg infantry can also field SP-Art. Putting all divisions on offensive supply can also help to tip the scales in your favour. Flying interdiction and logistical strike missions with your bombers will also help "soften up" your opponents.
Should you encounter a particularly well defended beach, try attacking in waves. Immediately after the first attack fails, have the next group of marines attack again, until the defences wear out. It's very costly, but taking land by amphibious assault always was.
Lastly, remember that having a capable surface action group (or MAF) in the sea zone adjacent to the province attacked. Not only will these ships protect your transports, but the shore bombardment they can provide is another (max) 25% combat penalty for the defender of the province. Also note that the shore bombardment will help your marines once they have occupied the province, giving the same penalty to the counter-attacker!
Planning the landing
Put together your landing force -- preferably marines with artillery and load them onto your transports. An Amphibious Assault mission is now active for the transports -- do not use it! It is bugged and can lead to lots of headaches. Instead, move the transports to a sea zone next to the province you want to attack. Click on the "unload" button of the transport fleet. Chose the units you want to use in the attack and right-click on the province you would like attacked. The attack will now function just as a normal land-based attack. Do not move your transports, as the attack will break off. Do not forget to put your fighting ships on Shore Bombardment mission.
Changes with Armageddon 1.3 Beta
For the Armageddon 1.3beta patch, the above does not hold. One can no longer disembark units into enemy territory. If you use the 1.3beta patch, you will have to use the amphibious assault mission. On the plus side, all remaining bugs with this mission type have been squashed and it should work exactly as intended. If you wish to attack in waves, this means that you will have to make a separate fleet for each wave, as the amphibious assault mission is fleet-dependent.