Japan had several reasons for their policy of expansionism. First and foremost was that they wanted to be as powerful as the Western nations that opened Japan up in the first place. They wanted to be treated as an equal or superior power by China, who traditionally had looked down on Japan. They also wanted to have a larger market in which to trade their wares and provide for the ever-increasing population.
The Sino-Japanese war of 1894-5 was basically caused by Japan's policies of expansionism, as outline above. This is because Japan wanted to possess Korea as an extension of the country, and if that was impossible, then at least coax it to break off its ties with China. With this in mind, Japan set about splitting China and Korea apart. This caused friction in all three countries, especially in Korea. Politics became very important, and as Korea became more and more under Japanese influence, China became more and more agitated. Eventually, a group of Korean militants rebelled, and the King of Korea asked China for assistance. Japan, unasked, also sent troops, and the two armies met in Korea. Japan refused to withdraw, and when the King did not cut diplomatic relations with China himself, Japanese forces imprisoned he and his family and appointed a puppet regent to do it for them. When this happened, China bubbled over and war was declared a week later.
The Chinese were never really in the race. They lost continually, and their army was pitched out of Korea at the speed at which it entered it - very fast. The Japanese army followed the Chinese over the border and across into Manchuria, and soon stormed the Port Arthur naval base. The navy was equally useless; although the Chinese possessed more ships than the Japanese, they were not as skillful at using them. On top of this, official corruption ensured that most forces had little ammunition to fight with. After a major defeat, the Chinese collapsed and surrendered soon afterwards.
The Treaty of Shimonoseki was signed, with China doing all of the giving and Japan all the taking. China's concessions included: 1) Korea was recognised as an independent state; 2) Japan gained the Liaotung Peninsula, Taiwan, and the Pescadores Islands; 3) four more trading ports were opened to the Japanese; 4) Japan gained a "most-favoured" status, which is what they were after; and 5) China had to pay 200 million taels.
Most Japanese were severely angered by the Triple Intervention. This document, signed by Russia, France and Germany, called on the Japanese to give the territory they had won back to the Chinese and accept 30 million instead of 200 million taels. Japan was not powerful enough to take on all three powers, and so backed down and did what the nations wanted. This put a chip on the shoulder of the Japanese as a nation, and they were especially bitter toward Russia.
The Russo-Japanese war of 1904-5 was basically caused by Japan being a bully in Port Arthur. Without warning, the Japanese attacked the Russian fleet at anchor in Port Arthur, causing great damage. Two days later war was declared.
The Russians did not expect to be beaten. They had virtually unlimited manpower, and their numbers swamped Japan's. However, the army was poorly equipped, the lines of communication stretched 5000 miles, and there was only one supply line. On top of this, the Russian population did not want the war as they thought that the government could be better put to use in fixing up internal problems.
The Japanese looked excellent. They had relatively short lines of supply, and they were well guarded. Both the army and the navy were new, well equipped, and courageous. The war plans were properly prepared, and the population were eager to get back at the Russians for their treatment in 1895.
The Russian navy had hassles from the start. The Japanese made sure that the fleet stayed within Port Arthur, stopping its usefulness, and any that managed to get out were eliminated. The Russians sent their Baltic fleet to help out the Pacific fleet - this took eight months to get there and when it did was destroyed in under an hour by the Japanese.
The army fared no better. Defeat followed defeat, culminating in the defeat of the Russians by the Japanese in March 1905. However, the strain of war was beginning to show. In Russia, the populace were becoming increasingly agitated by the defeats and a general dissatisfaction with the Tsarist system in general. In Japan, the government was worried over manpower and financial levels. Both sides wanted peace, and accepted an offer by U.S. President Roosevelt to mediate in a peace treaty.
The Russians came out worst in the peacemaking - they were forced to formally recognise Korea as Japan's; they surrendered the Liaotung Peninsula; and they lost the railway line between Mukden and Port Arthur. The Japanese also got half of an island. Both countries decided to withdraw from Manchuria.
Japan was the main instigator of this treaty. This is because the country saw Russia's intentions in the east - complete control for Manchuria and even Korea. These ideas were in direct opposition that Japan had, and so in seeking a western ally, in the form of Britain, was actually insuring against single war with Russia.
The Alliance stipulated that if either country was attacked by any two others then the other country was bound to help them out. Neutrality would be held if it was only one power. The agreement held for nearly 20 years.
Japan had already occupied and virtually run Korea for many years before the actual annexation. When the event occurred, it was almost, but not quite, a formality.
In 1906, Ito Hirobumi was appointed by the Japanese as foreign affairs minister. He received little co-operation from the Koreans, as they hated him, and so he requested that they be given executive and legislative powers. This was unacceptable to the King, and so he (the King) abdicated. The new King did accept, and this turned Korea into a virtual Japanese colony. When Ito was assassinated in 1909 by a patriot, Japan used the issue to force an annexation. They occupied the country for 35 years, when in 1945 they had to give it up as a war loss.