Camouflage.  
Camouflage was first introduced during WW1. As aerial assault became a threat, soldiers began looking for ways to hide from view of planes. They started wearing clothes which matched their environment. Today, camouflage is a major part of our military success. The most advanced camouflage that we currently have is worn by military snipers. Each sniper specially prepares their camouflage before every mission, to match the environment they will be in. They have suits where grasses and shrubbery can be placed in order to turn them virtually invisible from the enemy.

A sniper in his Ghillie suit. Even from a few feet away he disappears into the trees.