Huanggutun Incident (Chinese: 皇姑屯事件; Japanese: Zhang Zuolin Assassination Incident (張作霖爆殺事件 Chosakurin bakusatsu jiken?)) was an assassination plotted by the Japanese Kwantung Army that targeted Fengtian warlord Zhang Zuolin. It took place on June 4, 1928 at Huanggutun Railway Station near Shenyang in which Zhang's train was destroyed by an explosion. This incident was concealed in Japan at the time and was referred only as "満州某重大事件" (Manshuboujudaijiken A Certain Important Incident in Manchuria).
Following the Xinhai Revolution of 1911, China dissolved in spontaneous devolution, with local officials and military leaders assuming power independent of control by the weak central government. In north China, the once powerful Beiyang Army split up into various factions after the death of Yuan Shikai in 1916. Zhang Zuolin, being the leader of the Fengtian clique, was one of the most powerful warlords as he managed to seize control of the three northeastern provinces of Manchuria.
At the time of the First United Front in 1924, the foreign support in China was generally divided as below:
The Fengtian clique's support from abroad was the Empire of Japan, which had vested economical and political interests in the region since the end of the Russo-Japanese War, and was interested in exploiting their region’s largely untapped natural resources. The Japanese Kwantung Army, based in the Kwantung Leased Territory also had responsibility for safeguarding the South Manchurian Railway, and thus had troops stationed in Manchuria, which provided material and logistic support for the Fengtian clique. The cooperation initially worked to the mutual benefit of both parties. Zhang provided security for the railroad and Japanese economic interests, suppressing Manchuria’s endemic banditry problem and allowing extensive Japanese investments. The Imperial Japanese Army assisted Zhang in the two Zhili-Fengtian Wars, including the suppression of the anti-Fengtian uprising by Guo Songling (a senior Fengtian clique general). However, Zhang just needed Japan's aid for consolidating and expanding his territory whereas Japan envisioned a future joint occupation of Manchuria with Zhang. After Zhang achieved his targets, he tried to improve relations with the United States and the United Kingdom, allowing both countries open access to the trade, investment and economic opportunities in Manchuria which he had formerly allowed only to the Japanese. 
This change in policy came at a time when Japan was in the midst of a severe economic crisis from the Great Kanto Earthquake and successive economic depressions, and caused both alarm and irritation in the Kwantung Army leadership. The situation further complicated by the success of the Northern Expedition led by Chiang Kai-shek of the National Revolutionary Army in which the Kuomintang successively defeated Sun Chuanfang, Wu Peifu and other warlords of the Northern Faction, as well as the Beijing government controlled by Zhang Zuolin. The Nationalist army appeared poised to restore their rule in over Manchuria, which was still officially claimed as part of the Republic of China.
The Nationalists, the Communists and other elements in the Northern Expedition at the time were supported by the Soviet Union, which had already made puppet governments in nearby Mongolia and Tannu Tuva.
From the Japanese perspective, for Manchuria to fall either under Soviet or Nationalist domination was strategically unacceptable, and Zhang Zuolin no longer appeared trustworthy as an ally capable of maintaining a de facto independent Manchuria. Japan needed a context to effectively establish control over Manchuria without combat or foreign intervention and they believed splitting up the Fengtian clique via replacement of Zhang with a more cooperative leader would do so.
Outline of the Incident
Zhang left Beijing to go to Shenyang by train on the night 3 June 1928. The train traveled along the Jingfeng Railway, a railway that was heavily patrolled by his own troops. The only location along the railway that was not under Zhang's control was a bridge several kilometres east of Huanggutun railway station (in the outskirts of Shenyang), where the South Manchuria Railway crossed the Jingfeng Railway on the bridge.
Colonel Daisaku Komoto a junior officer in the Kwantung Army, believed that the assassination of Zhang would be the most expeditious way of replacing with a new leader more amenable to Japanese demands, and planned an operation without direct orders from Tokyo. His subordinate, Captain Kaneo Tomiya was in charge of executing the plan. The bomb itself was planted on the bridge by Sapper 1st Lieutenant Fujii Sadatoshi. When Zhang's train passed the bridge at 5:23 a.m. on the 4 June 1928, the bomb exploded. Several of Zhang's officials including Wu Junsheng (吳俊升), the governor of Heilongjiang province died immediately. Zhang was mortally wounded and sent back to his home in Shenyang. He died several hours later.
At the time of the assassination, the Kwantung Army was already in the process of grooming Yang Yuting (楊宇霆), a senior general in the Fengtian clique, to be Zhang’s successor. However, the actual assassination apparently took even the Kwantung Army leadership off guard, as troops were not mobilized and the Kwantung Army was unable to take any advantage by blaming Zhang’s Chinese enemies and using the incident as a casus belli for a Japanese military intervention. 
Instead, the Incident was soundly condemned by both the international community and by both military and civilian authorities in Tokyo itself. The emergence of Zhang’s son Zhang Xueliang (張學良/张学良) as leader of the Fengtian clique, and successor of Zhang Zuolin also came as a surprise.
The younger Zhang, in order to avoid any conflict with Japan and chaos which might provoke the Japanese into a military response, did not directly accuse Japan of complicity in his father's murder, but instead quietly carried out a policy of reconciliation with the Nationalist government of Chiang Kai-shek, which left him as recognized ruler of Manchuria instead of General Yang Yuting. The assassination thus considerably weakened Japan's political position in Manchuria. 
Furthermore, the assassination, which was conducted by low-ranking officers, did not have the prior consent of the Imperial Japanese Army General Staff Office or the civilian government. In fact, Emperor Hirohito harshly criticized the event and eventually dismissed Prime Minister Tanaka Giichi for his inability to arrest and prosecute the plotters of the incident, although privately he accepted the military's argument that doing so would be disadvantageous to Japan's military and foreign policy 
In order to achieve its goals in Manchuria, the Kwantung Army was forced to wait several years before creating another incident to justify the Invasion of Manchuria and subsequent establishment of the puppet state of Manchukuo under Henry Puyi Aisin Goro.
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- Bix. Hirohito and the Making of Modern Japan. pages 216-218 Zhang was a de jure head of state because he held absolute power over the internationally recognized Beiyang government based in Beijing.