This is the first part of my contribution to this episode. I will write about the rest(the KMT-CPC split and the Chinese Civil War) later.
The Warlord Era
After the failed attempt of a monarchy restoration under Yuan Shikai and the Constitutional Protection War, China was split between the North, under the Republic of China, and the South, under various warlords. Even the Republic had infighting amongst various groups. Duan Qirui and the Anhui Clique hoped for a military solution against the warlords in the South, while Feng Guozhang and the Zhili Clique wanted a negotiation.
After the Treaty of Versailles, German concessions in Qingdao were given to Japan instead of China. Many students protested against this in the May 4th Movement. This eventually led to the foundation of the Chinese Communist Party.
In October 1919, the Anhui-dominated government of China invaded and occupied Mongolia for several years until Ungern von Sternberg came. The North collapsed between the Anhui, the Zhili and the Fengtian(a clique in Manchuria). After the occupation of Mongolia, the Fengtian clique felt threatened and signed an alliance with the Zhili clique.
In July of 1920, war broke out between the Zhili and the Anhui. The alliance was confirmed, when the Fengtian also invaded. The two allies jointly occupied Beijing, until they betrayed each other.
(1922)In the first Zhili-Fengtian war, the Zhili won, and only stopped through British intervention, leaving the Fengtian only in Manchuria.
(1924)In the second war, the Zhili were almost victorious, when Feng Yuxiang launched a coup against the clique in Beijing. He established the Guominjun and the Fengtian clique won. The Zhili clique remained alive in Central China.
(1925)After the death of Sun Yat-Sen, Chiang-Kai-Shek took control of the KMT, which as based in Canton. He made sure to improve the military and received support from the USSR.
(1925)The Guominjun attempted to invade the Zhili and the Fengtian, and failed. (1926)In the South, the KMT found an opportunity to invade with their large military. They invaded from Canton towards the North, and captured most of the Zhili clique until they reached Nanjing. (1927)There, political instability prevented the KMT from pushing forward. Sun from the Zhili clique attempted a counter-attack, but failed and was pushed back.
(1928)As the KMT approached Beijing, the Shanxi clique became loyal and assisted with the battles. Zhang Zuolin, the leader of the Fengtian clique, was assassinated. His successor, Zhang Xueliang, was loyal to the KMT and signed an armistice with them. The KMT was now dominant.