He expanded Goguryeo’s territories far into the Korean peninsula by advancing southward at the expense of the Baekje dynasty to occupy the north of the Han River, and occupied Manchurian territory to the east of Liaohe. On his death in 413, at just 39 years of age, Goguryeo ruled everything between the Sungari and Han Rivers. This gave it control over two thirds of what is now modern Korea as well as a large part of Manchuria. In addition, the chieftains of Silla submitted to the northern kingdom's authority in 399 to receive protection from Japanese raids. Only Paekche continued to resist Koguryo domination during this period, thereby preventing what would have been the first recognised unification of the Korean peninsula.
During his reign, King Gwanggaeto conquered 65 walled cities and some 1,400 villages, in addition to aiding Silla when it was attacked by the Japanese. In 392 he built nine Buddhist temples in Pyongyang. His accomplishments are recorded on a monument which was erected in 414 in southern Manchuria.