I've read that during the Qing Dynasty, the Manchu set up a line of trees to be the border between the Han parts of China and the Manchurian ones and that Han were not allowed to cross it. Then the Japanese actually made it a totally different country. But fast forward to the end of WW2 and it seems as though it were completely Han.
So, what happened?
In the late 18th century the Manchus abolished the old barrier keeping Han immigration out of Manchuria. Initially as means of reliving bad economic conditions in northern China. By the 1840s urban Manchuria already had a Han majority, and the Manchus encouraged further immigration in the aftermath of losing outter Manchuria to the Russians because having a large ethnic Chinese population there was a way of preventing foreign powers from claiming further territory. By 1900 population in Manchuria had increased from 1 million the century prior to 14 million, largely because of Han immigration.
Han Chinese settlement in Manchuria began long before WWII. The Qing government loosed the migration restrictions and resettled large numbers of Han Chinese farmers in Manchuria during the later half of the 19th century. This migration continued throughout the late Qing and into the Republican/Warlord/Nationalist period. By the time of the Japanese occupation in 1931, the region was already majority Han Chinese.