In the Song dynasty military weakness, the decision is innate. before Song Dynasty was established, China was split and chaotic situation, the North and the South China separated, a number of constantly changing regime, war and turmoil, chaotic situation had been lasted more than 200 years.
It was at that time, the Qidan people in northern China to establish a powerful nation - Liao, and the opportunity occupied a large swathe of territory in northern China, these are the territory of the strategic northern China, including the Great Wall had been Liao occupation.
Song dynasty is in such an environment was established, from its beginning in the establishment was under military threaten of a strong Liao nation, the establishment of the early Song Dynasty, and had repeatedly fighting operation with Liao,attempt to regain the occupied northern territories, the famous Generals of the Yangs were all died for fighting during this period.
after long period of war, the Song had been at a disadvantage, lost more than it won after,the state consumed enormous, an important military reasons, the Liao monopoly of the North China's horses,Lack of horses in the Song Dynasty, it is difficult to resist the offensive Liao Cavalry, lost their military superiority, and finally the Song with Liao signed a peace treaty, contribut to the Liao each year a lot of money.
and worse is, in northwestern China, the Xixia also took the opportunity powerful, and the war also between Song and Xixia ,Song Dynasty army suffered a crushing defeat,20,000 soldiers were killed,the rest fled. Song Dynasty was forced to sign the final peace treaty with Xixia, the annual contribution of lot of money to the Xixia.
From then on the Song dynasty military was going to weakness.
Song Dynasty political darkness, corruption, and stirred up the people to continually resistance and revolt, the Song increasingly frail, finally, Nuzhen people( Jin ) occupied the northern territories of Song Dynasty,and captured emperor of the Song Dynasty, imprisoned deferred to the north, who were abused to death.
Song dynasty retreated south of the Yangtze River, that is, shame on the history of the Southern Song dynasty.