Complex sentence: 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, with the final battle being the relatively short naval Battle of Yamen in 1279.
Split into simpler sentences: After the Battle of Xiangyang, the Song Dynasty cannot enjoy the protection of natural barriers any more and so it collapsed in just a few years. The final battle was the relatively short naval Battle of Yamen in 1279.