The military history of Iran dates back millennia. Even before settling in what is now Iran, Indo-Iranian tribes had invented the chariot by around 2000 BC. During their expansion into Iran and Central Asia during the early first millennium BC, Iranians developed the concept of cavalry, being the first people in the world to ride horses in battle on a large scale. Ancient Iranians were also noteworthy for the participation of women in battle; up to 20% of some tribes' warrior graves have female remains. Also see Iran.