Historically, crossbows played a significant role in the warfare of East Asia and Europe. The earliest known crossbows were invented in the first millennium BC, not later than the 7th century BC in ancient China, not later than the 4th century BC in Greece (as the gastraphetes). Crossbows brought about a major shift in the role of projectile weaponry in wars, such as during Qin's unification wars and later the Han campaigns against northern nomads and western states. The medieval European crossbow was called by many names, including "crossbow" itself; most of these names derived from the word "ballista", an ancient Greek torsion siege engine similar in appearance but different in design principle. The traditional bow and arrow had long been a specialized weapon that required considerable training, physical strength, and expertise to operate with any degree of practical efficiency. Many cultures treated archers as a separate and superior warrior caste, despite usually being drawn from the common class, as their archery skill-set was essentially trained and strengthened from early childhood (similar to many cavalry-oriented cultures) and was impossible to reproduce outside a pre-established cultural tradition, which many cultures lacked. In contrast, the crossbow was the first ranged weapon to be simple, cheap and physically undemanding enough to be operated by large numbers of untrained conscript soldiers, thus enabling virtually any military body to field a potent force of crossbowmen with little expense beyond the cost of the weapons themselves.