The Northern Han State was weak, but it turned to the Khitan and was willing to be a vassal of the Khitans, while the Khitan regarded the Northern Han Dynasty as a reliable buffer between itself and the Northern Zhou-Northern Song Dynasty, and did its best to support its existence, whether it was Chai Rong or Zhao Kuangyin, had to face such a serious problem, that is, the war between itself and the Northern Han was actually equivalent to the war between itself and the Khitans, and without solving its own problems with the Khitans, it would be impossible to solve its own problems with the Northern Han.
In the first year of the Later Zhou Dynasty (957 AD), Chai Rong took advantage of the great victory in the Battle of Gaoping to attack the Northern Han Dynasty, and once attacked the nine prefectures of the Northern Han Dynasty (the Northern Han Dynasty had a total of nine prefectures and forty-seven counties, that is to say, all the prefectures were occupied by the Northern Zhou), and besieged Jinyang, but a month later, the Liao army came to the aid and broke the Zhou army Shi Yanchao's troops at Xinkou, and the Later Zhou army was attacked by the enemy and had to retreat, and all the prefectures and counties were lost.
In the first year of the Northern Song Dynasty (968 AD), the second year, and the ninth year of Zhao Kuangyin's three campaigns against the Northern Han Dynasty, the Northern Song Dynasty defeated the Northern Han army and even besieged Jinyang, and the result was also because the Liao soldiers came to the aid, or even only claimed to come to the aid, or threatened to attack all parts of Hebei, and had to withdraw their troops, at that time, the Southern Tang and other separatist forces were still there, the Northern Song Could not concentrate all its forces, plus a number of tactical mistakes (water attacks were self-defeating, camping in gancao fields caused soldiers to get sick), the result was still fruitless.
In the fourth year of the Taiping Revival (979 AD), The reason why Song Taizong was able to succeed in attacking the Northern Han Was first of all that the southern tang, the largest separatist force in the south, had been eliminated, and the only two remaining local separatist forces of Wuyue and Qingyuan did not have the will and ability to resist the Northern Song, and the Northern Song could devote the strength of the whole country to attack the Northern Han, and more importantly, at this time, the Khitan was successively ruled by Muzong Yelüjing, who was known as the "Sleeping King", and Jingzong Yelüxian, who was weak and sick, and the internal contradictions intensified, and they lacked the determination to rescue the Northern Han as in a big way as before, and later re-integrated the Khitan and later reintegrated the Khitan. At this time, Xiao Yu (Empress Xiao), who had demonstrated great ability, was not yet in full fledgling, and Zhao Guangyi, whose ability was far inferior to Chai Rong and Zhao Kuangyin, seized this gap and took advantage of the situation to attack the Northern Han Dynasty without a large-scale decisive battle with the Liao soldiers.
This kind of case of not being able to do it on its own, but surviving for a long time because it is a vassal of a powerful country, is not uncommon in ancient and modern China and abroad. For example, during the fall of the Liang Dynasty during the Northern and Southern Dynasties, the Northern Zhou Dynasty originally established Xiao Chen, the king of Yueyang of the Liang Dynasty, as emperor (historically known as Later Liang or Western Liang), this "Liang" area was only five states at its largest, and in fact it governed 8 counties and 16 counties, facing the repeated sieges of the Chen State, which had replaced Liang and called it emperor, but this weak projectile state relied on the protection of Northern Zhou and the later Sui Dynasty, and existed peacefully for 33 years, and finally was actively abolished by the Sui Dynasty because of an accident.