The Balhae (Parhae) kingdom in Manchuria (698-926 CE) was an important regional power which interacted both peacefully and otherwise with its neighbours the Unified Silla Kingdom of Korea and Tang China. The latter was a strong cultural influence, and Japan, too, was a staunch ally. The kingdom claimed to be the cultural successors to the earlier Goguryeo (Koguryo) kingdom of northern Korea and Balhae prospered for two centuries thanks to its rich agricultural lands and thriving trade relations. The kingdom collapsed in the early 10th century CE when it was attacked and conquered by the Mongolian Khitan tribe.
This is a companion discussion topic for the original content at https://www.worldhistory.org/Balhae/.