Japan’s relationship with China had been deteriorating for some years.  One particular reason was both nations’ claims to Korea.   Russia had an interest in Manchuria that neighbored the Korean peninsula.  A religious revolt in Korea touched off a chain of events that eventually led to the first Sino-Japanese War in 1894. 

Japan overwhelmed China and the Treaty of Shimonoseki (1895) settled their differences.  Among many things, China was forced to cede to Japan the Liaoning peninsula in Manchuria China.  Soon after the treaty, Europe got involved (known as the Triple Intervention) and forced Japan to give back to China the Liaoning peninsula.  

Russia reasserted its interest in Manchuria.  A humiliated Japan did not look back and launched an unprecedented industrial revolution and military buildup.  Japan had no intention to permanently abandon its interests in the Korean peninsula.