Excerpts from Books and Wikipedia
"The Battle of Xiangyang was a key battle between the invading Mongols of the Yuan dynasty and Southern Song forces from AD 1267 to 1273. After the battle, the victorious Yuan forces pushed farther into the Song heartland. Previously for 30 years, the Song dynasty managed to handle several major offensives by the Mongol Empire. The strategic significance of Xiangyang came from the fact that it was in a position dominating the Han river. Once the Yuan forces occupied Xiangyang, they could travel by ships down the Han river into the Yangtze river. After the Battle of Xiangyang, the Song dynasty could not enjoy the protection of natural barriers any more and so it collapsed in just a few years . . . Chinese firearms and cannons were employed by the Mongols in the victorious siege of Fancheng after capturing the outposts and relieving Chinese forces from Sichuan and Yuezhou, which broke through the siege but was eventually defeated. The use of the counterweight trebuchet by the Mongols proved especially effective." [Wikipedia] "Mongol victory broke the main defense of the southern Sung dynasty, leading to the establishment of the Yuan dynasty."