Koguryo was the largest, most powerful, and most aggressive of the three kingdoms. Due to its location, its main role from the first to the third century AD was to act as a bulwark against Chinese colonial forces and the aggressive, nomadic tribes that wandered the northern region. The survival of the kingdom depended on the ability of its people to defend themselves against constant attack and to preserve their territory. The Koguryo people lived in mountainous areas ill suited for agriculture, so they turned their hunting activities into a professional military way of life. The people of Koguryo were hardy and strong willed, and became militarily powerful with a united spirit of invincibility. Early references to Koguryo reveal a people who were fierce fighters that frequently engaged in warfare. They were described by their Chinese neighbors as being ambitious, aggressive, warlike, and courageous. For centuries, the people of Koguryo withstood invasions and defended their territory against all attacks.
As Chinese military colonies began pushing their way onto the Korean Peninsula, they brought with them advanced farming techniques and they introduced bronze to the peninsula in 108 BC. All this led to a rapid growth of both weaponry and farming for the Korean people. However, from the beginning Chinese rulers faced many uprisings by the Korean people against their rule. Starting from a point along the Hun River (a tributary of the Yalu), Korean rebels expanded their activities to the north, south, and southeast, increasingly menacing Chinese authority.
In the spring of 109 BC, the Chinese invaded the northern Korea peninsula and established a Chinese administration at Nang-nang, which endured for 400 years. The Chinese had great influence during this period and their presence led to the unification of many of the local tribes. In 37 BC, on a tributary of the Yalu River, a group who considered themselves a branch of the Puyo people united to form the kingdom of Koguryo under the rule of King T'aejo (53-146 AD). From its inception, Koguryo was dedicated the expulsion of the Chinese colony at Nang-nang.