During the First World War, Allied soldiers needed a way to see without being seen.
Concealment and deception had always been a part of warfare. But during the First World War the proximity of opposing trenches and the advent of aerial photography made it much easier to detect enemy troops.
Armies on all sides needed to find new ways to hide from, observe, and deceive enemy forces. The pioneers of those techniques were the French.
The solution was the camouflage tree, a fake piece of shrubbery with an observation post hidden inside. The camouflage tree would allow Allied soldiers to keep track of enemy movements from a concealed elevated position.