Why was the Song Dynasty's military always so weak?

Nov 2014
Why did the Song Dynasty have such a weak military compared to the other major Chinese dynasties like the Tang, the Han, or even the Ming and the Sui?
Feb 2017
Latin America
You're talking about the same state that gave one of the best performances militarily against the Mongols, when the Mongols were ruling an empire of about 15 million square kilometres extending from Mongolia to Muscovy (and which devastated Hungary like nothing). It took the Mongols decades to conquer the Song, and as if that wasn't enough, the Mongol Empire broke after conquering the Song. Offensively speaking, the Song may have been nothing special, but defensively they were very powerful.