THE UNITS

Infantry

The most common rookie mistake is without doubt not providing enough infantry. This unit is the backbone of any army, and should represent at least 80% of your total land forces, if not more. Whenever you're on attack or defense, they are always selected first as casualties, so your opponent has to clear them out before he has a chance to get to the armor and planes. So don't be stingy with the infantries when you make your purchases: in fact, you will typically spend all your revenues on them on most turns.

Armour

The best way to use your armour is to merge them all into one big pile, in anticipation of the big attack against your opponent's main forces. Never send one or two armor in those minor attacks against one or two infantries: they will be stuck in your new territory and you stand to lose them on your opponent's turn. Another use for your armors is for strafing tactics: it is simply put sending a massive attack force on a territory without taking it: you allow for one or maybe two rolls of dice and then retreats (typically when you start running out of attacking infantry). Then you do it again on your next turn. As a rule of thumb, your attack force should consist of an equal number of infantries and armours (and/or planes), and should outnumber your opponent's by at least 25%. The idea is to inflict more casualties than you take, so as eventually to wear out your adversary's forces by way of attrition. Be careful of being too successfull for you own good though, and seeing all those armors getting stranded without infantry support.

Fighter

Be always careful where you land your planes: you should always plan a couple of ground troops for fodder in your landing territory. In general, all your planes should be landed on the same territory, or split in two or three groups depending on the circumstances. Never ever land a fighter all alone where your opponent's ground troops can reach it: even 1 infantry is enough if he sends air cover. The main use of fighters is to provide defense for your main armies. On attack, they are essential in those trade-in-trade-out situations where two opponents will swap a territory back and forth each turn (for example, Soviet Union captures Ukraine with 2 or 3 inf, then Germany takes it back on their turn, then Soviet Union takes it back again, and so on). The best way to do this is by sending a couple of infantry (1 to 5, as needed) and provide cover with fighters and bombers. They are also useful for attacking ships, but make sure the odds are on your side.

Bomber

Most of what applies to fighters goes for bombers: keep in mind though that they are very poor on defense, and their high cost make them a very valuable target indeed, making it worth the risk of sending planes against it even without ground support.
A word about strategic bombing raids: before you figure you're gonna bring your opponent on his knees by bombing his factories, you might want to consider this: a bomber gets 1 die, giving an average of 3.5 on any single raid. Since a bomber can expect 5 successful raids before being gunned down by anti-aircraft guns, this translates as 17.5 total industrial certificates lost for your opponent as opposed to 15 for you (the price of a bomber); and you might get unlucky and lose it on your first run. The bottom line is while bombing raids is not necessarily a bad idea, the results are marginal, and it's generally more advantageous to use your bombers for supporting your attacks. Do make bombing raids if your bombers has no other attack to do, or if your opponent has entrenched all his forces in his capital and you want to wear him down before going for the kill, or if you can reach an industrial complex that is not protected by an anti-aircraft gun. Otherwise, don't spend all your ressources purchasing bombers to send them on bombing raids, it's not really worth it, and your revenues will be much more efficiently spent on ground troops. Unless of course, you happen to possess the Heavy Bombers weapon development...

Battleship

The most expensive unit in the game, and one of the most overrated. While a battleship provides great attack and defense for the fleet (its ability to support amphibious landings being a marginal advantage at best), its prohibitive cost makes it more than likely that you will have to make do with the ones you start the game with. If you ever need to beef up you naval forces, you might consider purchasing a carrier instead.

Aircraft carrier

Less expensive than battleships, carriers give your fighters extended coverage and thus a critical tactical advantage. Also, a fully loaded aircraft carrier is an impregnable fortress of the sea. Both of those reasons make carriers a better choice than battleships as the flagship of your naval forces.

Transport

Transports are the rank and file of the sea. While they have no attack value, they can be used as fodder to protect the carriers and battleships. But their greatest use is of course to be able to convoy land forces overseas, which is in fact the whole point of building a navy. You never seem to have enough of them; but transports by themselves are too vulnerable, and you have to provide them cover with either battleship or carrier; so if you want to land troops near your opponent's air force, make sure you have enough transports and other to discourage any thought of an air strike from his part. Note also that land-based attacks are generally more efficient than amphibious ones, so if your opponent is reachable by land, you might want to forget about building a fleet altogether and resolve to walk the distance instead.

Submarine

Submarines are in my opinion the most useless unit in the game. They only get a chance to attack on the first turn, and from then on the surviving ones will be mainly used as floating shields. Their ability to strike without reply looks great at first, until you realize that you'll almost never get the chance to move them within striking distance. The reason why the submarines are so worthless is their fatal vulnerability against air attacks: they can't strike back, and their withdrawal capability gives them a 50-50 chance to get away in the best of cases. In short, if you want defense for the fleet, buy transports instead, and if you plan to attack enemy fleets, buy planes.

Industrial complex

The first thing to remember before purchasing an industrial complex is always to ask yourself if transports might not be a better option; remember that transporting infantry is often more cost-efficient than building them, especially if your opponent has bombers nearby which will force you to purchase anti-aircraft as well. On the other hand, this argument doesn't apply to armor. But the most important consideration is that in all cases you have to ask yourself whether your opponent has any chance of capturing that new factory before you have a chance of reinforcing it with sufficient troops; losing a newly build industrial complex could very well spell disaster, as your opponent will now be able to build troops of his own from it; capturing an enemy industrial complex is always a big step towards victory in any case, so be wary of handling that kind of gift away. Don't forget to put an anti-aircraft too, as your opponent will always welcome any opportunity at a risk-free strategic bombing raid.

Anti-aircraft

Keep in mind that anti-aircraft can be moved around; they can be carried by transport too. Learn to use this to your advantage: by moving the aa gun from Southern Europe to Eastern Europe for exemple, you can support that large force you have stacked there, and will probably force your opponent's bombers to pass over it if he still wants to send a bombing raid in Italy. Another trick, that factory in Japan is more often than not out of range of enemy bombers, so consider the possibility of transporting the anti-aircraft and unloading it where you just have built an industrial complex in Manchuria or Burma for exemple; likewise the US aa gun can be brought to Finland. In short, keep viewing your anti-aircraft as mobile units.