As the Hongwu Emperor, first of the Ming Dynasty, however, he issued the first sea ban in 1371. All foreign trade was to be conducted by official tribute missions, handled by representatives of the Ming Empire and its "vassal" states. Private foreign trade was made punishable by death, with the offender's family and neighbors exiled from their homes. A few years later, in 1384, the Maritime Trade Intendancies ("Shibo Tiju Si") at Ningbo, Guangzhou, and Quanzhou were shuttered. Ships, docks, and shipyards were destroyed and ports sabotaged with rocks and pine stakes. Although the policy is now associated with imperial China generally, it was then at odds with Chinese tradition, which had pursued foreign trade as a source of revenue and become particularly important under the Tang, Song, and Yuan. The Treasure voyages of Zheng He were partly intended to monopolise overseas trade under the government. They were discontinued due to a rise in Mongol assertiveness after the Emperor's capture at the Battle of Tumu in 1449. The large scale of private overseas trade had caused price competition for the Ming government's purchases, such as warhorses for the northern frontier, and funds had to be reallocated. However, after the end of the Treasure voyages, Chinese trade within Asia and North Africa continued. Private, including unauthorised, Chinese trade in Southeast Asia expanded rapidly in the second half of the Ming dynasty. A 1613 edict prohibited maritime trade between the lands north and south of the Yangtze River, attempting to put a stop to captains claiming to be heading to Jiangsu and then diverting to Japan. Although the policy has generally been ascribed to national defense against the pirates, it was so obviously counterproductive and yet carried on for so long that other explanations have been offered. The initial conception seems to have been to use the Japanese need for Chinese goods to force them to terms. The Hongwu emperor seemed to indicate that the policy was designed to prevent foreign nations from collaborating with his subjects to challenge his rule; for instance, Srivijaya was banned from trading as the emperor suspected them of spying. The usage of trade was also a powerful tool to entice foreign governments to abide by the tributary system and pressure uncooperative leaders. Parallels with Song and Yuan measures restricting outflows of bullion have led some to argue that it was intended to support the Hongwu Emperor's printing of fiat currency, whose use was continued by his successors as late as 1450.