the next year as he had to avoid conflict with Japan to prepare for the conquest of Goguryeo. Tang dynasty The Tang dynasty (618-907) represents another high point for China in terms of its military might, conquest and establishment of vassals and tributaries, foreign trade, and its central political position and preeminent cultural status in East Asia. One of the most ambitious rulers of the dynasty was Emperor Taizong (r. 626–649). He initiated several significant war campaigns in Chinese history, most of them against powerful Turkic groups of Central Asia. This includes campaigns against Eastern Tujue, Tuyuhun, and the Xueyantuo. Armies were dispatched to invade the oasis states of the Tarim Basin. The kingdom of Karasahr was captured by Tang forces in 644 and the kingdom of Kucha was conquered in 649. The western expansion of the Tang Empire continued under Emperor Taizong's successor, Emperor Gaozong, who conquered the Western Turks in 657, led by the Turkic qaghan Ashina Helu, with an army under the command of General Su Dingfang. In a formidable alliance with the Korean kingdom Silla, a combined Tang-Silla fleet made a decisive victory over the Korean kingdom Baekje and her Yamato Japanese allies in the naval Battle of Baekgang in 663. Emperor Taizong also invaded Goguryeo in an effort to help their Unified Silla ally crush its rival, Goguryeo, to the north. Emperor Taizong's other intention in invading northern Korea was to secure territory of Lelang Commandery, an old Chinese commandery in northern Korea that had been lost since Goguryeo captured it from the Jin dynasty in the 313. However, Goguryeo's territory fell into the hands of Silla and Balhae instead of the Tang Empire. Chinese trade relations during the Tang dynasty was extended further west to the Arabian Peninsula, East Africa, and Egypt. Many contemporary writers from foreign countries described Chinese ships, Chinese goods brought to foreign ports, as well as Chinese seaports. Amongst the Chinese authors, the writer Duan Chengshi (died 863) described trade in Somalia and between 785 and 805 the Chinese geographer Jia Dan described lighthouses that were erected in the Persian Gulf, confirmed later by Muslim writers al-Mas'udi and al-Muqaddasi. The introduction of Islam in China began during the reign of Emperor Gaozong (r. 649–683), with missionaries such as Sa'd ibn Abi Waqqas, a maternal uncle to the Prophet Muhammad. The seaport at Guangzhou in southern China became one of the