Zhang Xueliang in the context of "Northern expedition"

The Northern Expedition was a military campaign launched by the National Revolutionary Army (NRA) of the Kuomintang (KMT) against the Beiyang government and other regional warlords in 1926. The purpose of the campaign was to reunify China, which had become fragmented in the aftermath of the 1911 Revolution. The expedition was led by Generalissimo Chiang Kai-shek, and was divided into two phases. The first phase ended in a 1927 political split between two factions of the KMT: the right-leaning Nanjing faction, led by Chiang, and the left-leaning faction in Wuhan, led by Wang Jingwei. The split was partially motivated by Chiang's Shanghai Massacre of Communists within the KMT, which marked the end of the First United Front. In an effort to mend this schism, Chiang Kai-shek stepped down as the commander of the NRA in August 1927, and went into exile in Japan.

The second phase of the Expedition began in January 1928, when Chiang resumed command. By April 1928, the nationalist forces had advanced to the Yellow River. With the assistance of allied warlords, including Yan Xishan and Feng Yuxiang, the nationalist forces secured a series of decisive victories against the Beiyang Army. As they approached Beijing, Zhang Zuolin, leader of the Manchuria-based Fengtian clique, was forced to flee and was later assassinated shortly thereafter by the Japanese. His son, Zhang Xueliang, took over as the leader of the Fengtian clique, and in December 1928, announced that Manchuria would accept the authority of the nationalist government in Nanjing. With the final piece of China under KMT control, the Northern Expedition concluded successfully and China was reunified, heralding the start of the Nanjing decade.

The Xi'an Incident was a Chinese political crisis that lasted from 12 to 26 December 1936. Soldiers of the Northeastern Army under the command of General Zhang Xueliang arrested Chiang Kai-shek, the leader of the Nationalist government of China, while the latter was in Xi'an to review the troops. Zhang demanded that Chiang agree to a ceasefire in the Chinese Civil War so that the Nationalist government could ally with the Chinese Communist Party (CCP) against Japanese expansionism. Negotiations were held between Chiang and the CCP, resulting in verbal agreement on the broad outlines of an alliance. After Chiang was released, he publicly renounced the terms he had agreed to in captivity, but secretly continued the negotiations that would result in the Second United Front.

The Xi'an Incident followed months of secret negotiations between the CCP and the Nationalists. Only minor progress had been made by December 1936. However, the CCP had also been negotiating directly with the Nationalist armies that surrounded it, including the Northeastern Army. These negotiations had been much more successful. The CCP formed a secret alliance with Zhang's Northeastern Army and Yang Hucheng's Northwestern Army that aimed to see the civil war ended and a war of national liberation begun against Japan. With encouragement from the CCP, Zhang repeatedly but unsuccessfully pressured Chiang to agree to a ceasefire with the CCP. After Chiang gave Zhang an ultimatum to either attack the Communists or be reassigned, Zhang decided to take Chiang hostage and force a settlement.

The Northeast Flag Replacement (traditional Chinese: 東北易幟; simplified Chinese: 东北易帜; pinyin: Dōngběi Yìzhì) refers to Zhang Xueliang's announcement on 29 December 1928 that all banners of the Beiyang government in Manchuria would be replaced with the flag of the Nationalist government, thus nominally uniting China under one government.

The Fengtian Government (Chinese: 奉系军阀; pinyin: Fèngxì Jūnfá; Wade–Giles: Feng-hsi Chün-fa) was the faction that supported warlord Zhang Zuolin during China's Warlord Era. It took its name from Fengtian Province, which served as its original base of support. However, the clique quickly came to control all of the Three Northeastern Provinces. The clique received support from Japan in exchange for protecting Japanese military and economic interests in Manchuria. The Fengtian Army frequently intervened in many of the conflicts of the Warlord Era.

Following the Zhili–Anhui War of 1920 and 1921, the Fengtian and Zhili cliques exercised joint control of Beijing and the Beiyang Government. Tensions soon began building between the two, resulting in clashes for control of Beijing known as the First Zhili–Fengtian War (1922). The Second (1924) Zhili–Fengtian War started later over the Zhili invasion of the remnants of the Anhui clique, which had become allies of the Fengtian Clique, which resulted in a Fengtian victory, with the Zhili clique retreating as far south as Henan. The power of the Fengtian Clique began to decrease in the midst of the Kuomintang's Northern Expedition. In 1928, while he was retreating North, Zhang Zuolin's Japanese sponsors blew up his train, killing him. After Zhang's assassination, his son, Zhang Xueliang, took over the leadership of the clique. He preferred to work with the Kuomintang rather than the Japanese and symbolically pledged allegiance to the Nationalist Government in Nanjing. In practice, the Fengtian clique continued to independently govern Manchuria until the Japanese invasion in 1931.

The Northeastern Army, also known as the Fengtian Army (see terminology), was a Chinese army that existed from 1911 to 1937. General Zhang Zuolin developed it as an independent fighting force during the Warlord Era. He used the army to control Northeastern China (Manchuria) and intervene in national politics. During the mid-1920s the Northeastern Army was the dominant military force in China, but in 1928 it was defeated by the Kuomintang's National Revolutionary Army (NRA) during the Northern Expedition. At the end of that campaign, Zhang Zuolin was assassinated and succeeded by his son Zhang Xueliang. When Xueliang subsequently pledged loyalty to the Kuomintang, the Northeastern Army became part of the NRA and was officially rechristened the "Northeastern Border Defense Force".

Despite being formally part of the NRA, the Northeastern Army remained de facto Zhang Xueliang's personal army. Zhang used the army to exercise considerable political influence during the tumultuous early years of the Nanjing Decade. The Japanese invaded Manchuria in 1931 and forced the Northeastern Army to retreat into northern China. After the army was unable to prevent further Japanese annexations of Chinese territory, Zhang was temporarily removed from command. In 1935, the army was reassigned to the Gansu-Ningxia border area in an attempt to encircle the Chinese Communist Party (CCP)'s base there. Both Zhang and his soldiers resented fighting fellow Chinese while Manchuria was under occupation. They negotiated a covert ceasefire with the CCP and hoped to convince Chiang Kai-shek to endorse a united front against Japan. After Chiang refused, the Northeastern Army kidnapped him and forced him to negotiate with the Communists. Although Chiang eventually agreed to end the civil war and work with the Communists against Japan, Zhang was placed under house arrest and the Northeastern Army was divided and reassigned to other commands.

Yang Hucheng (traditional Chinese: 楊虎城; simplified Chinese: 杨虎城; pinyin: Yáng Hǔchéng; Wade–Giles: Yang Hu-ch'eng) (26 November 1893 – 6 September 1949) was a Chinese general during the Warlord Era of Republican China and Kuomintang (KMT) general during the Chinese Civil War. He was a main supporter of Zhang Xueliang during the Xi'an Incident in late 1936, when the two generals plotted to force Chiang Kai-shek to cease hostilities against the Chinese Red Army and agree to a Second United Front against Japanese incursions into China with the Chinese Communist Party. In retaliation of his involvement in the incident, Yang was forced into exile by Chiang and then imprisoned by the Nationalist spy agency Juntong for 12 years, before being killed along with two of his children, his secretary Song Qiyun and Song's wife and youngest son in September 1949.