As the Jin dynasty failed to conquer the Southern Song, it faced the problem of defending two front lines both in the north and the south. Since its founding, the Jin dynasty had been suffering from severe financial difficulties, making it impossible to deploy standing armies along the extensive two-front lines with its weak national finances. Therefore, the Jin dynasty operated a mobilization system in which small standing armies would handle minor conflicts during peacetime, and reserve forces would be mobilized to counteract in emergencies such as large-scale wars.
Operating the mobilization system allowed the Jin dynasty to reduce defense costs and alleviate financial burdens during peacetime. However, during wartime, even production personnel had to be mobilized and sent to war, which lead to double financial hardships. Consequently, after transitioning to peacetime state in the mid-period of the dynasty, the Jin dynasty became extremely hesitant to engage in wars, missing opportunities in battles or causing invasions misjudged by the enemy.
The mobilization system required the standing armies to hold off the enemy during the mobilization preparation period, even in unexpected invasions . Therefore, the Jin dynasty established a triple defense system of trenches (界壕), fortresses (邊堡), and stronghold castles to conduct delaying tactics with a small number of troops against the fast-moving northern tribes. However, the Khitan, who played a significant role in northern defense, were excluded from the defense line due to distrust from Emperor Shizong of Jin. The vaccum resulting from that in defense capabilities was filled by the permanent occupation forces (臨潢等處永屯軍), tribal forces, and the Jiu Army (乣軍). However, the permanent occupation forces lost its defense capabilities due to its poverty, and the tribal forces and the Jiu Army gradually betrayed the Jin dynasty and defected to the Mongols. In the late period of Emperor Zhangzong of Jin, it is figured the northern defense line seemed to be effectively non-functional.
Thus, when the first Mongol invasion of Jin occurred in the third year of Daan (1211), the northern defense line’s system failed to conduct delaying tactics to buy time for mobilization preparations. The Jin dynasty was unable to carry out notable defense activities for about half a year until the mobilized army was assembled to counterattack, responding helplessly to the enemy’s invasion. The Jin dynasty, in the state of confusion and being defeated one by one, only engaged in earnest combat when the Mongol army rushed to the final gateway of the capital, Juyongguan (居庸關). After losing the only battle known as the Battle of Yehuling (野狐嶺) and Huaihebao (會河堡), the Jin dynasty was immediately driven to the brink of national collapse.