Hint: Make weak units useful:
As you press forward, you will capture cities and towns. Your units will also become weaker as they take damage. When a particular unit's strength is at 1, 2, or 3 (the number below the unit icon on the battle "map"), place the unit in a city you captured. Since the city has to be unoccupied for the enemy to reclaim it, your weaker units will become more useful, especially if the weaker unit is a tank force. However, make sure that you rebuild some units, as having all your weak units guarding a city is pointless; you should have no more than three or four in the city, so the stronger units can come back from the front lines to defend the captured city.

Hint: Anti-Aircraft Artillery:
The AAA (Anti Aircraft Artillery) is not only effective against planes, it also causes a lot of damage to infantry. Tanks can also be attacked, but the damage is not as good as against infantry.
Hint: Anti-Tank Artillery:
Anti Tank Artillery can only be used against tanks. It is useless against anything else.