It is thought likely that the first iron swords were manufactured in Japan in the fourth century, based on technology imported from China via the Korean peninsula. While swords clearly played an important cultural and religious role in ancient Japan, in the Heian period the globally recognised curved Japanese sword (the katana) was developed and swords became important weapons and symbolic items. The oldest schools in existence today arose in the Muromachi period (1336 to 1573), known for long periods of inter-state warfare. Three major schools emerged during this period.