The Evolution of Military Uniforms

Military uniforms have been in place ever since the first general wanted his troops to look imposing and cohesive on the battlefield. Uniforms were originally a way for all of the troops to look the same, and therefore feel more united as a team. Modern militaries generally stick with this strategy, and for much of the same reasons. By largely stripping incoming recruits of their individuality, military uniforms cause the recruit to see their identity as one of the cause, and not their own.
In ancient times up to about 100 years ago, military uniforms often had bright colors and distinctive designs. This made it easier for troops to recognize who were the enemy and who were friendlies, much like jerseys for modern football players. It also made it easy for troop commanders on a distant hilltop to survey the battle and instantly tell how the battle was going. Battles were often head-on, with large groups of troops lining up on either side (also like a modern football game). Strength of arms was important, but so was the power of intimidation. If one side could force the other to break ranks and run, the pursuing force instantly gained the upper hand. Brightly colored uniforms contributed to all of this.
However, while brightly colored, flamboyant uniforms might strike fear into the enemy and create a sense of solidarity among friendly troops, they are not very conducive to fighting an irregular war, that in which one side is significantly out-gunned by another. For this type of fighting, a head-on approach is usually suicide for the weaker force. Instead, that force will come at the enemy from the side or the back, often in small groups, striking quickly and retreating into cover. In this case, brightly colored uniforms are a liability. First, the element of surprise might be lost, since blending into the countryside is crucial to guerrilla warfare. Secondly, guerrilla warfare often relies on the larger force not knowing exactly who is an insurgent fighter and who is a civilian. Nondescript "uniforms" facilitate this deception.
A good example of one of the first instances of this mismatch in uniform strategies is the American War for Independence. The British, with their red coats and white, crossed sashes lined up and fired coordinated volleys in the forest, while the minutemen with their drab, buckskin clothing were able to hide and fire from behind cover. Uniforms with a more drab, ordinary look became more widespread among standing armies in the first World War, when trench warfare and machine guns made head on war all but impossible.
Today, many militaries have more than one uniform for troops. A formal, perhaps brightly colored uniform is for official functions during times or places of peace, when no fighting will occur, and a drab, durable uniform is for practical application in the field.