However, by the late 1930s the Kwantung Army's vaunted reputation was severely challenged during the Soviet–Japanese border conflicts that Japan had fought against the Soviet Union in northern Manchukuo since 1932. The Japanese force stalemated with the Soviet Union's Red Army in the Battle of Lake Khasan in 1938, and lost the decisive Battle of Nomonhan in 1939, during which time it sustained heavy casualties. After the "Nomonhan incident", the Kwantung Army was purged of its more insubordinate elements, as well as proponents of the "Hokushin-ron" ("Northward Advance") doctrine who urged that Japan concentrate its expansionist efforts on Siberia rather southward towards China and Southeast Asia.