For me, it is Heilongjiang, Jilin, and Liangning. These three provinces actually used to be six provinces under the Kuomingtang before 1949. Parts of the land there were lost due to invasions by Russia as well as the independence of the Republic of Mongolia. The ethnic Han in Dongbei actually have a very short history of living there. The Manchurians did not lift the ban on Hans immigrating there until the early 1890's. Granted there were Han who still did labor as well as specialized jobs but there were few. Most of the Chinese who live there now can trace their ancestry back to Shandong province. There was a massive wave of immigration from Shandong to Dongbei in the early 20th century. The Han who lived in Dongbei during the Ming Dynasty were killed off via Genocide by the Manchurians.