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TL;DR: If two homogeneous armies engage so that only a certain amount of units can attack at a time on both sides, e. g. in a choke point, then the outcome of the battle is decided by the armies' size multiplied by their relative unit strength multiplied by the number of units that can attack at a time. Whichever army has this value higher, wins the fight. Hence it is called the army's strength.
This theory works fairly well in real conditions as well.
More about Lanchester's Laws on Wikipedia.
Available in pdf format here.
As always, feedback is appreciated!

P.S. A post on Lanchester's Square Law coming soon.
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