In China, miniature models of ships that feature steering oars have been dated to the Warring States period (c. 475–221 BC). By the Han dynasty, a well kept naval fleet was an integral part of the military. Sternpost-mounted rudders started to appear on Chinese ship models starting in the 1st century AD. However, these early Chinese ships were fluvial (riverine), and were not seaworthy. The Chinese only acquired sea-going ship technologies in the 10th century AD Song Dynasty after contact with Southeast Asian djong trading ships, leading to the development of the junks.