During the reign of the Three Kingdoms, there was generally an open foreign policy. The prominent religion during this time was Buddhism and there was a great progression in the arts, architecture, and literature. Under the Silla kingdom, the Korean peninsula was united for the first time. The Joseon Dynasty succeeded after Silla. The mandatory religion during this period was Confucianism. The next 150 years, the country experienced peace and prosperity. One of the Korean kings during this time invented the script for the Korean language. Until this time, Koreans used the Chinese script. At the end of their prosperous period, a series of invasions began. Initially by the Japanese, then the Manchus. .
Exhausted from the invasions Korea became isolated. Worried by the growth of Catholicism and Protestantism and the occupation of Beijing by the French and British. They closed the doors to all foreigners and became known as the "Hermit Kingdom."" In the late 19th century this policy took its toll, while other countries were experiencing industrial and military growth, the Koreans were unprepared and fell to Japanese imperialism. The Japanese occupation ended at the close of World War II. At this time, Korea was separated into two nations. The USSR occupied the north above the 38th parallel and the US occupied the south, it was only meant to be a temporary border but failed negotiations led to the creation of two separate Korean nations. Soon after the withdrawal of Soviet and American forces, North Korea invaded the south thus initiating the Korean War. The North took Seoul in 3 days and advanced as far as Pusan, a coastal city on the far southeast of the peninsula. The United Nations Security Council decided to intervene. The UN forces, composed largely of American troops, pushed the North Koreans back near the Chinese border. China, fearing that the UN forces would not stop gaining ground at the Korean border decided to join in on the side of North Korea.