Complex sentence: However in 1259, the Mongols turned aganist the Southern Song and the Song was forced to capitulate and ceded all the territories north of the Yangtze River to the Mongols.
Split into simpler sentences: However in 1259, the Mongols turned aganist the Southern Song. The Song was forced to capitulate and ceded all the territories north of the Yangtze River to the Mongols.