After strengthening his government in northern China, Kublai pursued an expansionist policy in line with the tradition of Mongol and Chinese imperialism. He renewed a massive drive against the Song dynasty to the south. Kublai besieged Xiangyang between 1268 and 1273, the last obstacle in his way to capture the rich Yangzi River basin. An unsuccessful naval expedition was undertaken against Japan in 1274. Kublai captured the Song capital of Hangzhou in 1276, the wealthiest city of China. Song loyalists escaped from the capital and enthroned a young child as Emperor Bing of Song. The Mongols defeated the loyalists at the battle of Yamen in 1279. The last Song emperor drowned, bringing an end to the Song dynasty. The conquest of the Song reunited northern and southern China for the first time in three hundred years.