Invisible to enemy units that are not in an adjacent tile.
Whereas the sneaky barbarians had always used camouflage (of a sort, although the dirt and filth hid a lot), honorable soldiers didn't hide from the enemy. But the advent of the long-range musket and the Baker rifle made concealing oneself a survival necessity. During the French & Indian War the rifle-armed Rogers' Rangers wore gray and green uniforms, which blended in with the woodlands. During the Napoleonic Wars, the Portuguese Caçadores favored brown-jackets which helped conceal them. By the time of the First World War, all the major nations – save the French – had given up their red, white, and bright blue uniforms which made such lovely targets.