The Japanese occupation of Korea after annexation was largely uncontested militarily by the smaller, poorly armed, and poorly trained Korean army. Many rebels, former soldiers, and other volunteers left the Korean Peninsula for Manchuria and Primorsky Krai in Russia. Koreans in Manchuria formed resistance groups and guerrilla fighters known as Dongnipgun (Independence Army), which traveled across the Korean-Chinese border, using guerrilla warfare tactics against Japanese forces. The Japanese invasion of Manchuria in 1932 and subsequent Pacification of Manchukuo deprived many of these groups of their bases of operation and supplies. Many were forced to either flee to China, or to join the Red Army-backed forces in eastern Russia. One of the guerrilla groups was led by the future leader of communist North Korea, Kim Il-sung, in Japanese-controlled Manchuria. Kim Il-Sung's time as a guerrilla leader was formative upon his political ideology once he came to power.