http://www.quora.com/History-of-China/Why-did-the-Mongol-and-Song-alliance-collapse-after-defeating-Jin/answer/Kaiser-Kuo/comment/559277?__snids__=26514763#comment559282
That’s an interesting question. There were a number of reasons for their military weakness, despite their enormous army (Northern Song had 1.25 million under arms). Part of it was the fact that the military profession was denigrated, especially after the militarism that destroyed Tang. The army was made up of conscripts, and it was said that “good iron is not used to make nails, good men are not used to make soldiers.”