Cost: 3 I.PC.'S
Attack: 1 (unless supported by artillery)
Infantry is one of the most basic units in the game, and here are some strategies on how to use them. And remember, artillery support will increase infantry's attack.
Russia: The fate of Russia depends entirely on its infantry, if the didnt, Germany would beat them in less than two turns. Since Russia is on the defence most of the game, it would need all the help it can get. It also helps to buy infantry, since you start off as the poorest country, infantry is the only unit Russia can buy en masse. Infantry is really important to Russia and will serve them well.
Germany: With Germany being one of the more powerful countries in the game, infantry is more of an accessory. The place where infantry would be most helpful would be in Africa, since there is a little amount of troops on the continent for both sides, infantry should be spared there and let the tanks and other units smash Moscow or London. Infantry is also good for "policing" territories that can be taken by the Allies next turn.
UK: Britain is one of those countries that has a little of everything, but need infantry nonetheless. Britain's main concentration of infantry lies in the confindes of Asia and Australia. With these you can choose to either defend your territories, or take a bite at Japan.
Japan: Infantry is one of Japan's most valuable units. They are low costing and are easy to move around in the Pacific islands. They are the backbone of Japan's army but are second when compared to the navy.
USA: The US is isloated with in mainland America, but has troops in china as well. Use most of your infantry to fight Japan while your tanks take out the Germans.