The kingdom Sui(隋)/Tang(唐) period which organized common system and culture around china is important era in east asia. Unlike Sui(隋), Tang(唐) which build a continued unified kingdom is more important to the near country having relations with china. Specially, the founding and growth of Tang(唐) have close relations with the survival of the North-east countries and races including Koguryo(高句麗).
It’s easy to guess the importance of the mutual-assistance structure between Koguryo(高句麗) and Northern races to survive against the aggression of the unified china after Sui. The studying the relationship between Tang(唐) and these northern races from beginning of Tang(唐) to the fall of the Koguryo(高句 麗)(668) might be the one roundabout way to know the relationship between Koguryo(高句麗) and these northern races. Specially, after year 630（after the fall of DolGuel), the conflict between two powerful countries, Koguryo(高句麗) and Tang(唐), in north-east region is the matter of primary concern to these northern races, and their survival might depend on which side they are on.
During the early years of the Tang(唐), Tang(唐) have friendly relations with Koguryo(高句麗) because Dolgyul(突闕) still existed and Tang(唐) needed to overcome internal rebellion. Also, Koguryo(高句麗) might not need to build diplomacy relations with northern races because Tang(唐) was still in a state of disorder and Dolgyul(突闕) still existed.
But the year 630 when the Kahn Hilly(詰利可汗) was arrested, most northern races surrendered or established friendly relations with Tang(唐) and several races joined in Tang(唐)’s military forces to attack Koguryo(高句麗). Because of this situation, after year 630, Koguryo(高句麗) started to strengthen the diplomacy with northern races. As a result of this relations, Koguryo(高句麗) succeeded in Malgal(靺鞨) ,Seup(霫) and Silwy(室韋)’s not joining the member of Tang(唐)’s military forces to attack Koguryo(高句麗) in year 645, and lead Sulyounta(薛延 陀)’s alienation after failing of expedition to Koguryo(高句麗). Although there was Sulyounta(薛延陀)’s ruin from this alienation and Chulluk(鐵勒) tribal’s subjection to Tang(唐), Koguryo(高句麗) still strengthen diplomacy with northern races. Finally after year 660, Koguryo(高句麗) succeeded in making Keolan(契丹) and foreign forces ,which had been subjected to Tang(唐) after year 630, and Chulluk(鐵勒) tribal, which had been subjected to Tang(唐) after Sulyounta(薛延 陀)’s ruin(646), are alienated from Tang(唐).
These notrhern race’s alienation lead Tang(唐) not to make fully focus on attacking Koguryo(高句麗), and not to destroy Koguryo(高句麗) immediately after Bakjai(百濟)’s ruin. When I thought that these northern race’s alienation was not made by chance because these alienation was so helpful to Koguryo(高句麗), It’s proper that these alienation was from Koguryo(高句麗)’s continuous diplomacy effort with northern races. Naturally, we may think that Koguryo(高句 麗)’s strengthening of the northern diplomacy and northern race’s alienation and reorganization might have close relations with Tang(唐)’s central politic situation, but above all things, Koguryo(高句麗)’s strong military forces defending Tang (唐)’s frequent attack is more important fact to get the reliability from the northern races.