On 1 August 1886 (Meiji 19), the Qing dynasty's Beiyang Fleet, consisting of four warships, the Dingyuan, the Zhenyuan, the Jiyuan, and Weiyuan, entered the Nagasaki harbor port during a visit to various major Asian harbours.

[2] The Dingyuan had more tonnage than the heaviest Japanese cruisers in service, due to Japan's policy of following the Jeune École naval strategy, which emphasized small rapid assault craft.

In addition, Japan had recently suffered setbacks in the Gapsin Coup in which around 400 Japanese troops stationed in Joseon Korea were defeated by nearly 2,000 Qing-Joseon soldiers.

[citation needed] The Nagasaki Prefecture Police Department attempted to restore order with the help of a large number of local civilians.

Combined with the Gapsin Coup of 1884 (Meiji 17), the incident stirred up anti-Qing sentiment and was regarded as a distant cause to the First Sino-Japanese War.

The Qing's confidence was bolstered by the events of the Gapsin Coup where a small Japanese contingent was defeated by a much larger Qing-Joseon garrison.

A Japanese man named Wu Oogoro picked up a Beiyang Navy sailor's dictionary which was marked with 0-9 between the Chinese characters (Kanji).

In order to completely crack the code, Japanese Foreign Minister Mutsu Munemitsu deliberately provided a writing in Chinese characters of moderate length to the Qing ambassador Wang Feng Cao.