Ming fleet appeared onto the scene to restore peace, the fleet was defeated instead and the Japanese traders got away. After this episode, the Ming banned all Japanese from trading in Ningbo, who would have to join the pirates on offshore islands in order to trade Chinese goods. In May 1513, Portuguese explorers reached the coast of Guangdong, bringing the Columbian Exchange into China. This created a new global demand for Chinese products, while European firearms and New World crops were introduced to China for the first time. However, the potential of the China market was limited by the Ming's insistence of the maritime prohibitions. Early Portuguese attempts at trade negotiations were unsuccessful, and the Portuguese presence at the mouth of the Pearl River was driven off in the Battle of Tunmen of 1521 and the Battle of Xicaowan of 1522. The Portuguese, like the Japanese, were hence forced to conduct their trade illegally on the islands along the coast of China, away from the reach of the Ming authorities. Thus, both the Japanese and the Portuguese were left out of the tributary trade by the first decades of the 16th century. By the middle of the century though, Japan and Portugal became the premier suppliers of silver in East Asia by bringing in silver from mines such as the Iwami Ginzan in Japan and the Cerro de Potosí in Bolivia. Silver was the lifeblood of the Ming Chinese economy, but the Ming government placed many restrictions on mining due to fears that the bullion would pool into private hands. Even without the restrictions, China's silver veins were too small and located too far away in the southwest from the commercialized coastal provinces to keep up with the huge demand. This situation made unofficial dealings with foreigners very profitable and enticing for Chinese merchants, despite the risks and illegality of their trade. The "wokou" The term "wokou" literally means "dwarf pirates", with "wo" (倭; "dwarf") being the Classical Chinese pejorative for the Japanese people. The term was first used to refer to Japanese invaders on the Gwanggaeto Stele in 414 AD, and was reused in 1223 when Japanese pirates raided the Korean coast. These pirates eventually broadened their area of operations to the coast of China, and were active throughout East Asian coasts up to the establishment of the Ming dynasty in China. However, by the 16th century, the pirates