On March 28, 1894, a pro-Japanese Korean revolutionary, Kim Ok-gyun, was assassinated in Shanghai. Kim had fled to Japan after his involvement in the 1884 coup and the Japanese had turned down Korean demands that he be extradited.
On June 4, the Korean king, Gojong, requested aid from the Qing government in suppressing the Donghak Rebellion. Although the rebellion was not as serious as it initially seemed and hence Qing reinforcements were not necessary, the Qing government still sent the General Yuan Shikai as its plenipotentiary to lead 2,800 troops to Korea.
According to the Japanese, the Qing government had violated the Convention of Tientsin by not informing the Japanese government of its decision to send troops, but the Qing claimed that Japan had approved this. The Japanese countered by sending an 8,000-troop expeditionary force (the Oshima Composite Brigade) to Korea. The first 400 troops arrived on June 9 en route to Seoul, and 3,000 landed at Incheon on June 12.
Japanese foreign minister Mutsu Munemitsu meets with Wang Fengzao, the Qing ambassador to Japan, to discuss the future status of Korea. Wang states that the Qing government intends to pull out of Korea after the rebellion has been suppressed and expects Japan to do the same. However, China retains a resident to look after Chinese primacy in Korea.
Additional Japanese troops arrive in Korea. Japanese prime minister Itō Hirobumi tells Matsukata Masayoshi that since the Qing Empire appear to be making military preparations, there is probably "no policy but to go to war". Mutsu tells Ōtori to press the Korean government on the Japanese demands.
In early June 1894, the 8,000 Japanese troops captured the Korean king Gojong, occupied the Gyeongbokgung in Seoul and, by June 25, replaced the existing Korean government with members of the pro-Japanese faction.
Establishment of the Japanese Combined Fleet, consisting of almost all vessels in the Imperial Japanese Navy. Mutsu cables Ōtori to take any necessary steps to compel the Korean government to carry out a reform program.
Japanese troops occupy Seoul, capture Gojong, and establish a new, pro-Japanese government, which terminates all Sino-Korean treaties and grants the Imperial Japanese Army the right to expel the Qing Empire's Beiyang Army from Korea.
On 23 July 1894, Canadian Prime Minister John Thompson and his government made Labor Day, to be held in September, an official holiday. In the United States, the New York parade became an annual event that year, and in 1894 was adopted by American president Grover Cleveland to compete with International Workers' Day (May Day).
On 25 July 1894, the cruisers Yoshino, Naniwa and Akitsushima of the Japanese flying squadron, which had been patrolling off Asan Bay, encountered the Chinese cruiser Tsi-yuan and gunboat Kwang-yi. These vessels had steamed out of Asan to meet the transport Kow-shing, escorted by the Chinese gunboat Tsao-kiang. After an hour-long engagement, the Tsi-yuan escaped while the Kwang-yi grounded on rocks, where its powder-magazine exploded.
By 4 August, the remaining Chinese forces in Korea retreated to the northern city of Pyongyang, where they were met by troops sent from China. The 13,000–15,000 defenders made defensive repairs to the city, hoping to check the Japanese advance.
In early September, Li Hongzhang decided to reinforce the Chinese forces at Pyongyang by employing the Beiyang fleet to escort transports to the mouth of the Taedong River. About 4,500 additional troops stationed in the Zhili were to be redeployed. On September 12, half of the troops embarked at Dagu on five specially chartered transports and headed to Dalian where two days later on September 14, they were joined by another 2,000 soldiers.
Initially, Admiral Ding wanted to send the transports under a light escort with only a few ships, while the main force of the Beiyang Fleet would locate and operate directly against Combined Fleet in order to prevent the Japanese from intercepting the convoy. But the appearance of the Japanese cruisers Yoshino and Naniwa on a reconnaissance sortie near Weihaiwei thwarted these plans. The Chinese had mistaken them for the main Japanese fleet. Consequently, on September 12, the entire Beiyang Fleet departed Dalian heading for Weihaiwei, arriving near the Shandong Peninsula the next day.
On 15 September, the Imperial Japanese Army converged on the city of Pyongyang from several directions. The Japanese assaulted the city and eventually defeated the Chinese by an attack from the rear; the defenders surrendered. Taking advantage of heavy rainfall overnight, the remaining Chinese troops escaped Pyongyang and headed northeast toward the coastal city of Uiju. Casualties were 2,000 killed and around 4,000 wounded for the Chinese, while the Japanese casualties totaled 102 men killed, 433 wounded, and 33 missing. In the early morning of 16 September, the entire Japanese army entered Pyongyang.
The Chinese warships spent the entire day cruising the area, waiting for the Japanese. However, since there was no sighting of the Japanese fleet, Admiral Ding decided to return to Dalian, reaching the port in the morning of September 15.
Admiral Ding correctly assumed that the next Chinese line of defence would be established on the Yalu River, decided to redeploy the embarked soldiers there. On September 16, the convoy of five transport ships departed from the Dalian Bay under escort from the vessels of the Beiyang Fleet which included the two ironclad battleships, Dingyuan and Zhenyuan. Reaching the mouth of the Yalu River, the transports, disembarked the troops were and the landing operation lasted until the following morning.
On September 17, 1894, the Japanese Combined Fleet encountered the Chinese Beiyang Fleet off the mouth of the Yalu River. The naval battle, which lasted from late morning to dusk, resulted in a Japanese victory. Although the Chinese were able to land 4,500 troops near the Yalu River by sunset the Beiyang fleet was near the point of total collapse, most of the fleet had fled or had been sunk and the two largest ships Dingyuan and Zhenyuan were nearly out of ammunition. The Imperial Japanese Navy destroyed eight of the ten Chinese warships, assuring Japan's command of the Yellow Sea. The principal factors in the Japanese victory was the superiority in speed and firepower. The victory shattered the morale of the Chinese naval forces. The Battle of the Yalu River was the largest naval engagement of the war and was a major propaganda victory for Japan.
With the defeat at Pyongyang, the Chinese abandoned northern Korea and took up defensive positions in fortifications along their side of the Yalu River near Jiuliancheng. After receiving reinforcements by 10 October, the Japanese quickly pushed north toward Manchuria.
Describing their motives as having encountered a display of the mutilated remains of Japanese soldiers as they invaded the town, Japanese forces proceeded with the unrestrained killing of civilians during the Port Arthur Massacre with unconfirmed estimates in the thousands. An event which at the time was widely viewed with scepticism as the world at large was still in disbelief that the Japanese were capable of such deeds that seemed more likely to have been exaggerated propagandist fabrications of a Chinese government to discredit Japanese hegemony. In reality, the Chinese government itself was unsure of how to react and initially denied the occurrence of the loss of Port Arthur to the Japanese altogether.