Every schoolkid in China knows the story of the Manchurian Incident (九一八事變/瀋陽事變), a crucial part of China’s ‘Century of Humiliation.’ The narrative is easy: on 18th September,1931, a bomb exploded on a railway line near Shenyang (also known as Mukden in older sources). The Japanese Kwantung Army, under the influence of Ishiwara Kanji and Itagaki Seishiro, accused Nationalist China of setting off the explosion. In reality, the whole Incident was the brainchild of Ishiwara, Itagaki, and other members of the Kwantung Army. They intended to create a pretext to occupy southern Manchuria without notifying higher authorities in Tokyo. When the Kwantung Army sprang into action after the incident, Prime Minister Wakatsuki Reijiro was taken completely by surprise, and disputes over how to handle the incident led to the fall of his cabinet. Later governments, regardless of what was privately discussed, defended the actions of the Kwantung Army in public. There was no condemnation from Tokyo when Kwantung units pushed into northern Manchuria in November 1931, nor when the Kwantung Army achieved military control over the whole of Manchuria in February 1932.

However, Japanese interference in Manchuria did not start in 1931. We must trace it back to the First Sino-Japanese War in 1895, in which China was defeated. The Treaty of Shimonoseki, signed after the cessation of hostilities, ceded the southern part of Liaoning province (and de facto control over Korea) to Japan. Ten years later, the Russo-Japanese War encroached into Chinese-controlled Manchuria, but the Qing government was helpless in stopping fighting in her own territory. The end of the war saw Japan taking over Russian leases in China, including the Kwantung Leased Territory (hence, the establishment of the Kwantung Army) and perhaps more importantly, the southern branch of the Russian railway between Port Arthur and Changchun. This branch was developed into the South Manchurian Railway (SMR), which, much like the British East India Trading Company, had a variety of interests and held de facto control over the south Manchurian railway zone. One of the major responsibilities of the Kwantung Army was defence of the railway, which allowed it to expand in both influence and strength in Manchuria. In the chaotic warlord years of 1911 to 1928, Manchuria was kept relatively stable under the protection of the warlord Zhang Zuolin, who accepted Japanese aid but refused to fully collaborate. He was assassinated in 1928 by local Japanese in an action not approved by either Tokyo or the Kwantung Army, and replaced by his son (of Xi’an Incident fame), Zhang Xueliang. Zhang Xueliang was not popular with the Japanese, especially due to his decision to side with the Kuomintang (KMT). Negotiations between KMT and Japan in 1929 over the Manchurian issue did not go well, since the Nanjing government insisted on the evacuation of the Kwantung Peninsula as based on the original Russian lease of 1898, which would have expired by 1923. Not surprisingly, Japan rejected all claims. Minor incidents between Japanese constables and Chinese farmers further escalated tensions in 1931. The underlying conflict between Japanese and Chinese interests illustrated above, as well as blatant military insubordination, led to the Manchurian Incident.

There was no armed resistance against the Kwantung Army, because both Zhang Xueliang and Chiang Kaishek agreed that the best way to deal with the situation would be to appeal to international authorities. Much to the surprise of many, Japan agreed to a commission of foreign observers operating under the apparatus of the League of Nations. What the international community did not know was that Japanese Foreign Minister Shidehara Kijūrō was highly annoyed by the Kwantung Army and wanted international intervention to discipline its leaders. But before the commission arrived, the Kwantung Army had once again outmanoeuvred its nominal superiors and set up the last Qing Emperor, Pu Yi, as the head of an independent state, Manchukuo, on 1 March, 1932. After extensive fact-finding in Shanghai, Nanjing, Peking, Shenyang, Changchun, Harbin and Tokyo from March to July 1932, the commission published the Lytton Report (named after Victor Bulwer-Lytton, who headed the commission). Their conclusions were that:

1. The Manchurian independence movement ‘had never been heard of before 1931’ and ‘was only made possible by the presence of Japanese troops’
2. The real power behind the mainly Chinese Manchurian government rested with Japanese officials and advisers, who were more than happy to ignore instructions from both Tokyo and the Kwantung Army high command
3. ‘Military operations of the Japanese troops cannot be regarded as measures of legitimate self-defence,’ but at the same time, the commission ‘does not exclude the hypothesis that officers on the spot may have thought they were acting in self-defence’

According to Nish, the commission essentially wanted to condemn the new state of Manchukuo, ‘recognise China as the suzerain power in the territory, but give Manchuria a degree of local autonomy under which Japan’s treaty rights would be fully respected.’ Japan rejected the claims of the Report and walked out of the League of Nations. Hostilities did not cease between the KMT and the Kwantung Army until May 1933. Pushing south of the Great Wall, Japanese forces encountered heavy Chinese resistance, especially around Peking. Both sides decided to sit down for talks, resulting in the Tanggu Truce, which defined a demilitarised zone 30–40 miles wide between the Great Wall and a line north of Peking. As Nationalist China was neither politically unified or military prepared to take on the Kwantung Army, the Truce essentially held until events in 1937, despite constant border conflicts.

The Manchukuo government decided to adopt a planned economy, which was dominated by SMR, and later on, the Manchurian Heavy Industries Development Corporation (MHIDC). Manchuria proved a perfect fit for heavy industries, with its developed railways and abundant coal, mineral and iron ore reserves. Manchukuo did achieve some economic success, but on the whole, as reported by the British consul to Manchukuo, Paul Butler:

Manchukuo is designed solely to serve Japanese interests political, economic and military. The number of Japanese officials in the Government has increased steadily and now exceeds 50 percent of the personnel of higher grades. Persecuted incessantly by gendarmerie and police and deprived of the means of livelihood and opportunities for advancement by the great incursion of Japanese, the small Chinese educated class is in process of rapid disintegration. The inevitable outcome must be that all wealth and positions of responsibility will be monopolised by Japanese who will exploit an inarticulate Chinese peasantry. Judging from these indications and from the ambitious plans for Japanese and Korean immigration, one can only assume that it is intended to create a self-governing Japanese dominion rather than an independent Manchukuo State.

I will answer it as a timeline hoping to help you understand easier. It dates back to 1890s. The Manchuria here is the land outside Shanhai Pass, Outter Manchuria is not included as it will be just invasion and massacre by the invading Russian. Some events I type down the line might look like they have no relation but without them, some events happened later will become hard to understand. Since you ask things happened before WWII, I stop the progression on 1939 Use the map of Chinese Eastern Railway to understand better.

1895 - End of the First Sino-Japanese War. Treaty of Shimonoseki was signed. Article 2&3 China cedes to Japan in perpetuity and full sovereignty of Formosa, and eastern portion of the bay of Liaodong Peninsula (Dalin) together with all fortification, arsenals, and public property.

1896 - Li-Lobanov Treaty was signed in Moscow to counter the aggression of the Imperial Japanese. A railway(which is the very last end of the Tran-Siberian Railway)in Russian standard was built though Heilongjiang and Jilin to Vladivostok. Russian also owned the extraterritorial jurisdiction over a larger part of Manchuria and the permission to station troops to protect the railway.

1898 Convention for the Lease of the Liaotung Peninsula was signed. Port Arthur and Dalin and some inland were leased to Russian for 25 years as a naval base. Later signed another one, permitting for Russian to build the sideline of Chinese Eastern Railway from Herbin to Port Arthur

1900 - Russian invaded Manchuria in the name to protect the railway and her rights during the Boxer Rebellion

1902 - A treaty 交收東三省條約 (Wikisource in Japanese as I can't find anything about this in English) was signed for the Russian to retreat from Manchuria

1903 Russian was refused to retreat and reoccupied Mukden, Japanese negotiate with the help of Britain and the US, Russian who was still refused

1904 - Start of the Russo-Japanese War. Most of its battlefield was in Manchuria and the sea surrounding.

1905 End of the Russo-Japanese War. Treaty of Portsmouth was signed. The rights of the Russian in Laidong Peninsula and much of the South Manchurian branch to Japan. The rail line from Changchun to Dalin transferred to Japanese control, later became the South Manchuria Railway.

1906- Another treaty 中日會議東三省事宜條約 (Wikisource in Japanese as I can't find anything about this in English as well) was signed.

1911 -Xinhai Revolution, the formation of the Republic of China Provision Government in Nanking. Dr SUN Yat-sen was almost unanimously elected as the provision president. The war between the revolutionist and Beiyang New Army end with the help of YUAN Shika who also forced the Manchurian royal family to abdicate the throne. In return, Dr SUN steps down and YUAN became the provision president.

1912- Republic of China formed. YUAN became the first president. YUAN refused to make an oath in Nanking by causing a rebel in Peking with his Beiyang Army.

1913 Assassination of Song Jiaoren, leader of tongmenghui and the first parliament. The revolutionist blamed YUAN was behind this. YUAN also mortgaged the customs and salt tax to borrow money from the British, French, Prussian and Japanese and borrow money from an Austrian Armed company, all without the permission from the parliament. The revolutionist started the Second Revolution

1914 - YUAN government agreed with the Twenty-One Demands from Japanese The portion about Manchuria mostly included in Group II Under The Articles of Favourable Treatment of the Great Qing Emperor the royal was able to stay and living inside the Forbidden City

1916 YUAN died as he failed to establish The Chinese Empire. His commander in the Beiyang Army started a struggle to become the president of the Beiyang Government. Started of the Warlord Era. Manchuria under control of the Zheng family(Fengtian clique, a Manchuria based warlord) who is supported by Japanese.

1917- Dr SUN was leading the resistance against the Beiyang government by establishing another government in Guangzhou. Later known as the Constitution Protection Movement. February Revolution, end of the Russian Empire. Establishment of the Russian Republic October evolution, end of the Russian Republic. Establishment of the Russian Soviet Republic. The White Army control the Trans-Manchurian Railways. The Reds later gained control.

1920-Zhili-Anhui War started, Fengtian clique joined later and helped the Zhili won

1921 - Establishment of the military government from Dr SUN in Guangzhou

1922 - First Zhili-Fengtian War, Zhili won. Zhang Zuolin declared the autonomy of Manchuria

1923 - Declaration of Sun-Joffe Manifesto, the revolutionary government in Guangzhou was aid the CCCP. Establishment of the Whampoa Military Academy. Formation of the National Revolutionary Army led by Chaing Kaishek.

1924 Second Zhili-Fengtian War, Zhili Warlord Feng Yuxiang kicked the remaining royal out of the Forbidden City. Puyi went to Tianjin. Fengtien clique won and took over the Beiyang Government and Zhang Zuolin became the president of Beiyang Government

1927- Start of the Northern Expedition. CCP rebel and Chiang killed all communist in front of his march. Chiang seek for Weimar military help which was turned down as Chiang requested Ludendorff and von Mackensen, which against the Treaty of Versailles

1928 - NRA reached Peking, Zhang Zuolin, who was escaping back to Mukden, killed by the Japanese Kwantung Army with bombs in Huanggutun. Later known as Huanggutun incident. His son, Zhang Xueliang as successor, reconciliation with the Nationalist government. China "reunification" weakened Japan's political position in Manchuria.

1929 Sino-Soviet Conflict, lead by Zhang Xueliang, aimed to take back the Soviet control Trans-Manchuria Railways, failed badly.

1931 - Mukden Incident. Zheng Xueliang ordered not to engage. Manchuria has fallen into Japanese. Puyi went to Manchuria, hoping to restore the empire of Manchurian.

1932 Establishment of Manchukuo. Metropolitan Plan of Great Hsinking was drawn. 1933 Tangku Truce was signed, a demilitarized buffer zone was set up.

1933 Adolf Hitler was the Chancellor of Germany, who disregarded the Treaty of Versailles, sent Max Bauer to China as the Nazi and KMT are both anti-communist. [pic of preimer of ROC H. H. Kung with Hitler]{https://en.wikipedia.org/wiki/Sino-German_cooperation_(1926–1941)#/media/File:Kung_and_hitler.jpg} 1934 Puyi was crowned Emperor of Manchukuo, under the reign title Kangde in Changchun were later changed to Hsinking. General Han von Seeckt, as an advisor to Chiang, proposed an 80 Division Plan. Only 8 infantry division had completed reorganisation and training. Later known as Germany-trained Division as they wear German uniform and use Germany weapon. pic

1938-Puyi was declared a god by the Religions Law

Those three government coexist because Chiang didn't want China got caught in a war too early as the Japanese are way too advance in all way you can imagine. Chiang didn't declare a war on Japanese until 1941, after the attack the Peral Harbour. He planned to trained 80 german division to counter the Japanese as the Chinese army and unorganized and badly equipped. A declaration with Manchukuo or Japanese mostly mean embargo and losing help from Nazi. Which mean the end of the war. So Chiang "allow" them to exist. The CCP was also a reason the Nationalist Government can't put all effort to fight even during WWII. Zhang Xueliang and Yang Hucheng started the Xi'an Incident which the kidnap Chiang as it was the last strike on CCP. They only have a few thousand left. Chiang orders hundred thousand of troops surrounding the CCP. Chiang believed the Reds was a bigger threat to China than Japan. He once said "Japanese is skin diseases but Communist is heart attack" 10 years of Operations(1926-1936) against the CCP makes the other two "stay". Manchuria is the origin of the Qing, I can date back to 1600 but this would take lots longer. Be sure to let me know if you want to. Also keep in mind reunification in 1927 is on technical, just because the warlord was defeated. They run independently from the Nanking Government as Chiang is too busy even when in the middle military operation but keep getting supply from Chiang's government.

I hope i haven't miss out anything. I hope I solve your problem. :)