Right after their foundation, Sui and Tang, for about a period of seventy years, the two dynasties went through seven large-scale wars against Koguryo. As a result, Koguryo finally fell by the allied forces of Silla and Tang, and Silla defeated Tang’s armed forces.
Then, what factors made the war last for about seventy years and what caused both Sui and Koguryo to collapse in the end, despite such diplomatic endeavors of the two countries as frequently sending their envoys to each other. There should be some inevitable causes for Sui and Tang dynasties to bring about a war with Koguryo in terms of their international relations, given that they were not that fully ready for it right after their foundation with unstable political situations. This paper was to present a new approach to it while positing that some domestic matters of the two dynasties should be the fundamental cause of the war.
Sui-Tang dynasties’ wars against their neighbouring dynasties like t?yuhun, Koguryo are mainly for coping with Turk’s military threat. The war against Koguryo was one of the ways to confronting with Turk. Needless to say, Koguryo itself was a threatening force against Sui-Tang China. In addition, Koguryo was suspected of the political and economical alliance with Turk. Therefore Sui-Tang dynasties took military actions against Koguryo to get rid of one wing of Turk.
Sui-Tang dynasties developed into the world empire after unification of China. Sui-Tang dynasties were tried to reestablished ‘the tribute and vassal system’. This time they meant to apply this system not only to domestic but also to foreign kingdoms thoroughly. Neighboring dynasties were enforced to attend the variety of ceremony in the capital city of the empire just like as local government officials were. These ceremonies were designed for showing off the dignity and dominance of the dynasties over in and out of the empire. Both dynasties tried to strengthen the dominance over local forces through the extension of the system into the foreign kingdoms.
Besides, this system had a very different aspect from the old ones. Sui-Tang dynasties drew back the actual power of the local leader for the centralization of government. Technically, they were determined to apply this system to Koguryo. They didn’t write the actual ruling power of the king in the edict when they gave the title of the ruler to the kings of Koguryo. This was unacceptable for Koguryo. Sui-Tang dynasties didn’t yield to Koguryo in this point. They might think that they couldn’t guarantee the domestic stability without their dominant power over the neighboring kingdoms. This war continued because of Sui-Tang dynasties’ inner factor, ended with the fall of Koguryo.