Zhang Xueliang's seemingly superior force was undermined by several factors. The first was that the Kwantung Army had a strong reserve force that could be transported by railway from Korea, which was a Japanese colony, directly adjacent to Manchuria. Secondly, more than half of Zhang's troops were stationed south of the Great Wall in Hebei Province, while the troops north of the wall were scattered throughout Manchuria. Therefore, deploying Zhang's troops north of the Great Wall meant that they lacked the concentration needed to fight the Japanese effectively. Most of Zhang's troops were under-trained, poorly led, poorly fed, and had poor morale and questionable loyalty compared to their Japanese counterparts. Japanese secret agents had permeated Zhang's command because of his and his father Zhang Zuolin's past reliance on Japanese military advisers. The Japanese knew the Northeastern Army very well and were able to conduct operations with ease.