Since ancient times, most of the time. The division after World War II was because South Korea was originally controlled by Japan. After Japan surrendered, there was a power vacuum. Kim Il Sung was originally anti-Japanese in China at that time. He had an army and a mass base, so he was able to occupy North Korea. The United States fostered the "Korean traitor" during the Japanese rule to become the government so as not to let the peninsula belong to North Korea. The aborigines did not accept it, so there was the Korean War.