The Fall of the Xian Dynasty and The Rise of the Liu Dynasty
(1311 - 1451)
With the annezation of Northern Korea, the Xian Dynasty reached it's height in power in 1311. Unfortunately for them, their heyday would not last long. The empire had 2 enemies with great power as well, the Shang Kingdom, and Inner Mongolia. There were other nations in the land too, such as Japan, Tibet, Great Malacca, the Indian Empire, The Golden Horde, and of course, Korea.
Xian had had several disputes with the 2 kingdoms, but this was the worst. Both Shang and Inner Mongolia declared war on the empire. Combined, it was too much. Several areas collapsed, and were brought under control of either nation. Meanwhile other disputes occured in Tibet, who gained Nepal and Bhutan. The Indian Empire fell into civil war as well, and the nations of Mumbai and Madras arose. In the Pacific, Japan also fell apart, with Uesugi and Kagoshima rising up.
By 1374, the war was wearing down the empire quickly. Inner Mongolia conquered the link between Xian and Peking, and split the dynasty in two. Japan meanwhile collapsed completely, along with Madras. Great Malacca starts to move away from Tibet, who is also losing territory. Mumbai, too, falls back to the coast, and starts to collapse. Korea is reformed into Pyongyang, which takes back North Korea, and the land known as Shandong Bandao, which was breifly occupied by the Shang.
By 1399, all hope is lost for the empire. Xian is overrun by Inner Mongolia, and their ally, Pyongyang, loses part of North Korea and all of South Korea. Tibet and the Indian Empire fall apart, and Malacca loses all territory above the Malay Peninsula. But Xian's enemies are crumbling, too. The Shang are losing land to the west, while the Golden Horde and Inner Mongolia go to war, which hurts them alot. The Kingdom of the Dragon also becomes independent from the Shang.
In 1432, the Emperor dies without a son, and a succession crisis occurs. But a new Emperor rises to the throne, and begins the Liu Dynasty, which ends the Xian Dynasty. Inner Mongolia, the Golden Horde, and the Shang start to crumble completely, while the Dragon Kingdom takes some of the coastline and Pyongyang expands to Japan.
Notes: I don't know what Pyongyang and Dragon Kingdom think of this, but this is how I think they came to be.