Rouran base wasn't central Asian They base was what is today's Mongolia and Hinggan Mountain. So Rouran surely will attack you. And now you are well into in North china Wei already making alliance with Rouran. Around 400 they at the helm of their power so, noone can stop them.
Basically most fierce military in North East was Nomads on Mongolian Plain. What is the Chinese employed tactic is bribe them with huge bounty and use them against the other enemy of China.
There are different definitions regarding the specific limits of Central Asia. This
map shows three examples, in which one definition covers all of Mongolia and most of Western China. You can haggle over the specifics if you want to, but the fact is that throughout history, the states within the region as a whole were closely related to each other regarding several political issues, and often influenced each other. However, they were culturally and ethnically separate, generally speaking.
Anyway, your post only confirms my initial assumption that you either skimmed the posts, or ignored them altogether and focused on assumptions solely based on the situation OTL. In other words, please try to read the events within the timeline as thoroughly as possible. I specifically stated in a previous post that ITTL, the Wei suffered a
decisive defeat on Canhe Slope in 395, allowing the Yan to retain its relative hegemony in North China, and the Wei to rely more on other states. I then went further to state that the Wei was on the
verge of collapse before Murong Xi made plans to attack Goguryeo in 404, causing Gwanggaeto to respond within that year. The following conflicts freed the pressure on the western alliance, and eventually allowed the western and eastern alliances to converge and destroy the Yan altogether in 410. However, North China continued to remain divided among several entities until 417, when the Wei finally managed to gain influence over most, but not all, of the region, and will spend decades in order to further consolidate its rule and confront the holdouts to the west. In addition, the Rouran IOTL did not attempt to attack Goguryeo, and attempted to maintain more friendly relations with the Yan around 410, which was a rump state at the time, suggesting that it would be nowhere near consolidation around 400-20 either IOTL or ITTL. In addition, the Rouran would have to attack the Beili before even attempting to invade Goguryeo, and the latter would be prepared by that time for such an attack.
In other words, neither the Wei nor the Rouran would be even close to prepared when considering an invasion into Goguryeo, due to their relative weaknesses at the time. The Rouran would also be more concerned about North China, as it would view the Wei as a similar rival state that must be weakened in order for the former to extend its control.
Regarding to Wei, consolidating their control over China would be more easy than Gogureyo establishing control over Chinese proper. Since Wei-Xianbei will just adopt Chinese culture while Gogureyo will force they own culture.
So if you want to make Gogureyo stable you need to always make sure that Gogureyo and Mongolian Nomadic Empires was ally. So you direct them Against Power in North China and Western Expansion, while Gogurey will focus on consolidating his base on Manchuria, Hebei and Shangdong. However don't expect nomads follow they agreement. They will time to tiime raid your border and loot your cities.
I never stated once within this thread that Goguryeo will attempt to assimilate the Han Chinese in any way, shape, or form. The inhabitants west of the Liao River will be mostly left alone, culturally speaking, and political divisions will be based on cultural norms, similar to what occurred during the Liao and Yuan dynasties. Meanwhile, the Wei ITTL would be much more concerned with political consolidation, along with invading its neighbors to the west, which suggests incohesion for several decades.
Goguryeo will eventually ally with the Rouran, but the latter is nowhere near consolidated at this point to establish formal diplomatic relations. In addition, even when the Beili ceased to exist and Goguryeo allied with the Rouran, the latter was focused on attacking the Wei instead of raiding Goguryeo, due to the geopolitical situation at the time.
Economy doesn't work. North Chinese is now 2 century constant war and conflict. Even not counting natural disaster like drought and flood, the Economy and population is exhausted. Unless you claim Mandate of Heaven (which means adopting Chinese culture) the conquered people won't strength your army nor will they easily submit. So your troop expansion is unrealistically high.
Patience. This timeline is technically the third version, and it took me more than two years to find enough sources, and about half a year to finalize the posts within the timeline that you see here, not to mention revamping most of the details before. You're talking about issues that will occur centuries from my most recent update, although it took me several months to come up with the initial version of the events from 391 to 420. As a result, I only have vague outlines for the events that will follow centuries after the PoD.
That being said, there will be about 50 years of relative peace within North China after 417, as the Wei will control most of the region, and will maintain cordial relations with Goguryeo for decades as the former attempts to consolidate power. As a result, there will be plenty of time for the population within East Asia to recover as a whole, and when conflict eventually occurs, specific events that occurred during the peace will eventually lead the chaos to be much more limited, relatively speaking. Goguryeo will also probably claim the Mandate of Heaven around 450-500, and I thought that I made the basis for the troop expansion clear in my previous post.
If Goguryeo, which had a total population around 4-5 million, managed to gather 600,000 troops against the Tang, I don't see why they won't be able to gather an amount between 100,000-500,000 after it unifies the peninsula, although the actual figure will probably be around 100,000-300,000 when taking recruitment of the Han Chinese into consideration. Also, around 500-550, based on the extant censuses IOTL, the "Korean" population will probably be around 8-10 million, while the "Chinese" one will be around 30-35 million ITTL. The corresponding figures around 600-650 will be around 15-17 million and 50-60 million, respectively, although both will decrease significantly after the major war. In other words, adopting the Mandate of Heaven will barely affect Korean culture as a whole, due to the population ratios, and based on the fact that the Mongols, who were vastly outnumbered, mostly remained intact even after a century of rule over all of China, I don't see why the situation would be any different for Goguryeo.
