There's a timeline around here somewhere where Alexander the Great is overthrown while in Afghanistan or India. He and his loyalists do this giant Volkswanderung across Asia, intermarrying with some horse tribes, and end up settling in Korea, where they kill off a lot of the native male population during the conquest.
Alexander dies in battle in Japan fighting pirates.
I haven't read this TL, so I don't know the details, but the scenario essentially looks ASB to me. Although the Xiongnu were first recorded in Chinese sources around 244 BC, its possible that a consolidated state, or several consolidated tribes, existed around 350-300 BC. Later, the Xiongnu eventually grew into a powerful empire which fought the Qin and Han continuously for more than
two centuries. As a result, considering that Alexander never commanded more than 10,000 calvalry, and all of them would be dead or low on morale by the time he would come into contact with the Xiongnu, his army would almost certainly be decimated within a few weeks or months.
Even if he managed to hold out against the Central Asian tribes, it would be virtually impossible to cross the Pamir, Tarim Basin, and/or the Himalayas. These regions were some of the last to be settled in Asia due to their extremely inhospitable climates, and although Alexander managed to found Alexandria Eschate in the extreme northeast of his empire, he was forced to go further south, not east, because of the imposing geography. He would need to trek though thousands of miles of desert and/or mountains, and the tribes in the area would not take too kindly to a foreign army straggling by, so the soldiers would have to think about survival and holding out against various unknown hostile tribes before anything else. If anything else, Alexander's soldiers would desert him if he attempted to head further east.
The alternative would be settling down in Central Asia and gradually assimilating the natives around the area, although it's much more likely that the Macedonians would be assimilated instead due to their inferiority in numbers. However, even if Alexander dies when he is 400 or 500, this state would not make its way further west due to the Xiongnu, Xianbei, Turkic, Uighur, and Khitan tribes, who collectively dominated Central Asia for more than a thousand years. Even if he or his descendants manage to get past them somehow, he would have to confront either Goguryeo or Balhae, both of which dominated Southern Manchuria and the northern regions of the Korean Peninsula for more than 800 years from around 100-900. Considering that Goguryeo itself was also semi-nomadic, and its troops retreated east into the mountains after its capital was sacked
twice, and reestablished itself soon afterward, the state from the west would find it virtually impossible to subjugate Manchuria, let alone Korea.
Democracy101,
Though, you would lose Heaven Lake, it is not that hard to say the POD is Baekdu goes inert. Maybe there is some Earthquake after the previous major eruption that cause the crust to shift. This drains the Magma chamber.
I complete see and agree with you on the points about the Tocharians.
It looks like you know more about geological events than I do, so I'll take your word for it, although this just means that a unified "Korean" presence in Southern Manchuria would last, and that regardless of the eruption, even if the Tocharians somehow arrived in Manchuria, they would be unable to control it for more than two centuries.