Corrupt official's exploitation in Korea and Qing's interference
Before Tonghak Rebellion broke out, there were many officials sucking things out of peasants. Also, Qing's interference in Korea's status really annoyed people. This made people angry which later results Tonghak Rebellion.
(The date is not specific)
Due to the corrupt official's exploitation, and Qing's interference, people of Korea, especially the peasant class, got angry at the governent. People started a rebellion which is now called the Tonghak Rebellion.
Sino Japanese War
Qing Dynasty sent troops to Korea to settle down the Tonghak Rebellion. However, Japan took it as a violation of national rights of Korea and declared a war against Qing Dynasty. The important thing is that Japanese intentions were not to save Korea but to free Korea from the Qing so that they can take over Korea.
Treaty of Shimonoseki
Since Qing Dynasty had lost the war, they had to give up Korea and pay many things that Japan had demanded. This event is the treaty that Chinese had to sign, which made it possible for the Japanese to conquer Korea in earnest.
Treaty of Ulsa
Treaty of Ulsa was an unequal treaty that Korea was forced by Japan due to their invasion. It totally invaded Korea's national rights which brought the Japanese annexation of Korea.