History of Manchuria
Manchuria refers to what we nowadays often refer to as Northeast China (the provinces of Heilongjiang, Liaoning, and Jilin as well as a small part of Inner Mongolia). The term can also include parts of Russia that were once part of this region (these parts are referred to as "Outer Manchuria" in English). Although this region is nowadays often referred to and seen as an ethnically "Chinese" area with a Han Chinese majority (albeit with a diverse minority of Manchus, Mongols, Koreans, Evenkis, and others) that's often seen as China's equivalent to the "Rust Belt" areas of the USA, Manchuria is much more than this and the region wasn't even seen as part of China until the middle of the 20th Century. This region was-and still is-coveted for its fertile soil and rich reserves of raw materials and for this reason, the region was historically contested between Japan, Russia, the Manchus, China, and the Mongols.
The history of people in Manchuria dates back to Neolithic times (6200 BC). This is known from cultural sites with traces of life in the Neolithic era such as the Xinglongwa culture, Xinle culture, and Hongshan culture. In the times of antiquity, several dynasties of Ancient China (such as the Han, Cao Wei, and Western Jin) and Korea (notably the Kingdom of Balhae and the Goguryeo Dynasty) ruled parts of Manchuria. However, the region was mostly ruled by Tungusic peoples such as the Nanai, Ulch, and Mohe (who would later become the powerful Jurchen people) since the area was the ethnic homeland of the Tungusic ethno-linguistic family.
By the 12th Century, Manchuria was divided by the Mongolic Khitan people who ruled part of the region as the Liao Dynasty and the Tungusic Jurchens who ruled the area as the Jin Dynasty. The area was in a state of constant turmoil and the two Dynasties were also often at war with the Chinese Song Dynasty to the south. However, in the 13th Century, all of Manchuria, along with all of Korea and China, was conquered by the Mongols who established an empire that would stretch from Hungary to the Pacific Ocean. When the Mongol Empire was split into four separate Khanates, Kublai Khan took control of Mongolia, Manchuria, Korea, and most of modern-day China as the Yuan Dynasty in 1271. During this time, Manchuria was administered as Liaoyang province. After the Yuan collapsed and the Mongols were expelled from most of China, Manchuria came under the control of many Jurchen clans that fought amongst each other and against the Chinese-ruled Ming Dynasty that was trying to exert its influence into Manchuria (the Ming was partially successful as they controlled the southern part of the region, known as Liaoning).
In the 1580s, the Jurchen chieftain Nurhaci of the Aisin-Gioro clan started to unify the Jurchen tribes and by 1616 he'd declared himself the Khan of the Later Jin Dynasty which he established as a Jurchen state ruling Manchuria. Nurhaci subsequently started to resist Ming influence in Manchuria and started to drive the Chinese out of Liaoning. In 1618, the Jurchens launched a full-scale war against the Ming when they launched an invasion against them. In 1626, the Jurchens came under the rule of Nurhaci's son Hong Taiji. As the Jurchens conquered more territory, their ranks became more multiethnic with the absorption of Ming deserters, North Chinese, Mongolian, and other Tungusic peoples. This led to Hong Taiji's decision to change the name of his people from Jurchen to Manchu in 1636. The origin of this name is unclear, though one theory is that it's a combination of the Manchu/Jurchen words "mangga" (difficult, strong, or brave) and "ju" (arrow) which would make sense as the Manchus were feared and respected by other nations for their reputation as skilled archers. Hong Taiji also changed the name of his Dynasty from Later Jin to Qing because he wanted to both unite his nation and conquer the Ming while establishing a new dynasty to rule over China (in this era, "China" referred to only the areas populated by Han Chinese people). The Manchus also established an elite force of soldiers known as the Eight Banners ("Jakvn Gvsa" in Manchu) that would become instrumental in conquering new territories for the Qing as well as internal administration.
In 1644, the Manchus, under Prince Dorgon, conquered Beijing and this led to the deterioration of Ming forces until the final traces of the dynasty were extinguished in 1683, after the Manchu conquest of Taiwan. After the conquest of China was complete, many Manchus started to move to Beijing to cement their position as rulers of China and also started to adopt Chinese cultural practices such as the adoption of Chinese philosophies and the study of the Chinese language by many Manchus. However, the Manchus made efforts to preserve their culture through means such as renovating the Forbidden City in Beijing with Manchu signs and translating Chinese works into Manchu. They also forced some of their own practices on the conquered Chinese. Most notable was the requirement of Chinese men to shave their hair except for a part growing from the back of the head in a ponytail. This practice of wearing a ponytail in one's hair (known as "Soncoho" in Manchu) became known worldwide but became associated with China instead of Manchuria because it became known due to Chinese immigrants wearing it.
After the Manchu conquest of China, Manchuria was largely neglected due to the Manchu emperors' focusing on Beijing. However, after the Russian-Manchu border conflicts in the mid 17th Century, the emperors started to realize how important their homeland was and took measures to preserve it not only to act as a bulwark against Russian Imperialism but also to be used as a reserve of reliable Manchu soldiers that would always be loyal to the Qing (and also as a refuge in the case of the Qing being overthrown by the Chinese). For this reason, the Manchus banned Chinese immigration to Manchuria to ensure the protection of the Manchus and other Tungusic peoples in the region from Chinese cultural domination (as the Manchus in Beijing had become increasingly Sinicized). Nevertheless, this had to be balanced out with practical needs due to the low population density of Manchuria making the region vulnerable to Russian and Japanese expansionism (this was demonstrated by the Russian annexation of Outer Manchuria in 1860 as well as Japanese incursions into the region during the Sino-Japanese War and the Russo-Japanese War). This led to a gradual increase in the Chinese population and ultimately, the lifting of the immigration ban in the 19th Century which led to a mass migration of Chinese into Manchuria and the Manchus becoming a minority in the region. This led to the decline of the Manchu language.
When the Qing Dynasty was overthrown by Chinese revolutionaries in the Xinhai Revolution of 1912, the Qing emperors were confined to Beijing and many Manchus were persecuted by the Chinese throughout the former empire. The situation in Manchuria, however, was much better for them because even though the area now had a Chinese majority, it came under the rule of the Chinese warlord Zhang Zuolin who sympathized with the Qing and established the Fengtian Clique to rule the region while also protecting the Manchus. However, in 1928, Zhang Zuolin was assassinated by Japanese spies and his son Zhang Xueliang decided to cooperate with the Chinese government (thus making Manchuria a nominal part of China).
In 1931, after the Mukden Incident, the Japanese Empire invaded Manchuria and established the puppet state of Manchukuo (known as 満州国in Japanese). Although the Japanese claimed to create the state in the name of the Manchus, they made the official languages Chinese and Japanese and installed either Japanese or collaborationist Chinese officials into government positions and also massacred Manchus and Chinese indiscriminately. The only Manchu who was in any significant position in Manchukuo was Puyi, the last emperor of the Qing Dynasty (and the last Emperor of China) who was used as a puppet and didn't speak Manchu during his "reign" over Manchukuo. Japan eventually used Manchukuo as a springboard to launch a full-scale invasion of China in 1937. Japanese rule in Manchukuo and other parts of China under Japanese occupation was extremely harsh and the people faced brutality from both the Kempeitai (the Japanese secret police) and the massive Japanese Kwantung Army (関東軍) who often exploited the people in ways such as medical experimentation (most infamously in Unit 731 in Harbin).
In August 1945, after Atomic Bombs were dropped on Japan, the Soviet Union invaded Manchuria. After the war, the region became a base for Chinese Communists as they fought the Chinese Nationalists in the Chinese Civil War. Eventually, the region formally became part of China in 1949 when the Communists founded the People's Republic of China.
Due to Chinese immigration to the region over the years (which increased the population to the present 108 million), as well as persecution of Manchus and other groups native to the region during the Cultural Revolution, Manchuria nowadays has an overwhelmingly Han Chinese majority and the Manchu language is nearly extinct despite there being over 10.5 million Manchus. Manchuria nowadays is one of China's most industrialized regions although it still has many pristine areas. The population includes not only Chinese and Manchus but also sizable minorities of Koreans, Japanese, Evenkis, and Mongols. In recent times, many Manchus are rediscovering their roots and the Manchu language is being studied by many people in the region.