Japan wasn’t one of the key players in World War 1, but it did play a considerable role. An important question one may ponder upon is why did Japan join the World War 1 which majorly took place in the European Nations and what was Japan’s interest in joining the war.
So, the thing is that during the later half of the 19th century, Japan had transformed from a closed society into a military aggressive nation with expansive interests. They had taken part in a few wars before including the ones that they won against China(1894-95) and Russia(1904-1905).
Japan was somewhat allies with Britain but it wasn’t like the Japanese were obligated to join the war. Japan’s agenda was to take control of German small holdings in the Pacific region and the coast of China.
Japan was able to take control of some German possessions including Tsingtao and other Pacific islands. Japan also sent its military aid mainly naval fleet to help the Allied Forces far away in the Mediterranean.
Japan was able to be a part of the Paris Peace Conference with major help of the Allies and put forward their intentions during the Treaty of Versailles. Basically, Japan wanted an equal authority as the other European nations in the League of Nations.
Japan’s agenda was mainly to be elevated and recognized as a major world power.