During the Nara period of Japanese history, between 700 AD and 900 AD, there was a transition from having an imperial army comprised of conscripted men to relying more and more on specialized forces comprised of men who made war their trade. These were the first samurai, who began as mercenary forces hired by the imperial court. Long swords, known as katana, were primarily used by the samurai. The signature weapon of the samurai, the katana is worn edge up so that drawing and attacking can be combined in a single motion.

Source: Worcester Polytechnic Institute