Well, the American Revolutionary army tried that, at first, with their militia. They were copying the tactics that seemed to work during the French and Indian Wars. However, collections of individuals shooting from behind trees and such were generally only useful for harassment, not for taking and holding a battlefield.
Also, fighting in such an "ungentlemanly" manner caused the British troops to take out their frustrations on the unarmed citizenry of the towns they occupied ...
Eventually, the Continentals brought in "Baron" von Steuben to train them in the European style. The rest, as they say, is history.
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