Even if armor seems like a good idea for warriors heading into battle, some don't believe that it's necessary. In fact, there have been plenty of naked armies in history. The ancient Greeks had their dudes strip down before they fought in boxing and wresting matches, in line with the general nudity of the gymnasium. Similarly, the people of the Nuba Mountains in modern Sudan often wrestle naked as a sacred practice.
There were some warriors who fought nude for religious or psychological reasons. Berserkers hailing from Scandinavian cultures honored their god by stripping completely - they also freaked out their opponents by going commando. Over the generations, there has been lots of nudity in combat.
The early medieval Vikings were fierce fighters, so much so that some of them even fought in the buff! They were unafraid for their own lives, stripping themselves of tunic, chainmail, and even underpants to engage in combat. This was sometimes regarded as a form of holy insanity, a way of honoring a god of war in the purest form of worship by dedicating their bodies to him.
While the archeological evidence of berserkers is scant (just like their clothing), they were depicted artistically as starkers before they were even mentioned in sagas!
The Indian dashnami sannyasin are ascetics, people practicing "strict self-denial," who belong to of one of ten orders following the philosopher Sankara's schools of thought. They originated in the Middle Ages and still exist today. Often wearing colorful robes and tiger or leopard skins, the dashnami sannyasin don't all go naked. The ones who do are called naga sannyasin, who cast off clothing and usually walk around with only a weapon.
The naga sannyasin have fought with other Hindu sects or Islamic warriors; for them, the nude body symbolized renunciation before the divine.
In the third century BCE, the Romans sought to expand their power throughout the Italian peninsula and came to a head with their Celtic neighbors. Located in northern Italy, these rivals were a confederation of tribes that included a people called the Gaesatae. While their allies wore light armor, the Gaesatae, in "vanity and bravado," decided to go buck naked.
They reportedly believed that fighting naked would be more effective, since nothing would get in the way of their weapons. The sight of nude warriors wearing gold jewelry frightened the Romans, but the tribesmen found their nakedness ineffective in combat when the legions launched spears at them; some supposedly even threw themselves on their enemies' weapons in despair. After a prolonged struggle, the Romans finally emerged victorious, even after their leader was decapitated.
During the Vietnam War, some troops called sappers — who were responsible for engineering duties — were given suicide missions. These sappers stripped to just their underwear and snuck into enemy camps to deposit explosives.
These weapons were often strapped to their backs as they snuck beneath wires and past guards.