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.
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