Infantry are a good for a defense, because each costs only 3 IPCs and scores a hit on 2 or less when defending.
Tanks cost more than infantry (5 IPCs) but attack and defend on a 3.
They can also move farther than other land units and are very suitable for a blitzkrieg.
Fighters are strong on attack and defense but can fly only 4 spaces.
Bombers can fly up to 6 spaces and can do special strategic bombings.
But they are more expensive and much weaker in defense.
Battleships are very powerful, both in attacking and defending (a die roll of 4 or less).
They require 2 hits to sink, but cost 24 IPC's.
Destroyers are cheaper, attack and defend on a 3 and limit even enemy submarine capabilities.
Cheap submarines attack and defend on a 2, but they have a deadly opening fire shot and can move through hostile sea zones.
Aircraft carriers have strong defensive capabilities and allow your fighters to land far from shore.
- Transports carry land units to embattled territories but are sitting ducks for enemy attacks.
You should attack when you think you can win (and keep enough surviving units) a battle to gain more IPC's at the end of your turn.
Sometimes (even when you are weaker), it's a good idea (for sure as Russian) to attack your enemy to disrupt his strategy.
If you can retreat after scoring more casualties than you got yourself, it was an worthwhile attack.
Try to do strategic bombing raids with your bombers as much as possible.
The IPC loss will hinder your oppoment a lot.
Developing new weapons is expensive and risky because it doesn't guarantee anything. Consider to do it as German Player when your blitzkrieg is not super succesful.
The Allies must make sure that the Russians can hold position against the Germans. The game is usually won or lost on the Russian front.
This way they buy time and make use of their economical advantage.
The allies can choose to neglect Japan and focus to kill Italy or Germany first.
The Axis must get even in economical production.
If the Allies choose to neglect Japan, then Japan must advance towards Russia as soon as possible.