Tanhai

Tanhai is a country in East Alshar, bordering Corumm to the south, Canpei to the west, xxxx to the north, Metzetta to the northeast and the Ocean of Cathay to the east.

Antiquity
The Old Kingdom of Jin which spanned territory that is part of today's Corumm and Tanhai was founded by members of the Bao clan in approximately the year 220 BC. The noble Bao clan had escaped into southern Tanhai around the year of 300 BC after losing a power struggle in the state of Sui. Slowly rebuilding their political fortunes, they intermarried with the nomad clans of the Degei and gained their military support.

Nomad Migration and Imperial Province
Circa early 600's BC nomad tribes migrating(expelled by other tribes, fleeing famine/natural disaster/plagues/seeking new grasslands/etc) from the north Tanhai settle in northwestern Tanhai, create the Degei confederation(I originally imagined them as closely resembling the manchu people 'Degei' is just what they are called in Corumm). Circa 670's they start doing what nomads do and go raid and wreck other peoples shit, aka they ride south into the corummese shang dynasty. After a few years they get their shit pushed in and Shang moves into Tanhai and turns part of it into an imperial province(extensive corummese settlement at this point in south and central tanhai) and the Degei are relegated to the northern border zone as vassals and military auxiliaries.

Revolt and Despotate of Tang
Downfall of Shang dynasty in 840-850's spells doom for control of the province, ambitious viceroy declares himself Despot of Great Tang with foreign backing(Dia or metz local actors or central govs depending on situation at the time) and buys loyalty of local garrisons. Tang invades eastern peninsula to take it from the metzettan statelet, probably loses badly and wastes precious military resources in the process. Smelling weakness, nomads stir again. Despotate limps on several decades more

Kingdom of Tanhai
Tang name gets corrupted into Tanhai? or something lol. Northern nomads over time become landed nobles/marcher lords/sipahis providing safety at the border/cavalry for Tanhai's armies in exchange for land and other benefits

Demographics
A note on the Metzettan diaspora.

Economy
a quick word on the currency

Transportation
Tanhai uses the same Five Foot Six Inch Gauge track as Corumm, allowing their railway networks to be interconnected across the border.

road, air and sea

Communications
Tanhai's top-level domain is .th.

phones and postal service