Japan participated in the First World War from 1914 to 1918 allied with the Triple Entente and played an important role in securing the sea lanes in the Western Pacific Oceans and in the Indian Ocean against the German Imperial Navy. Politically, Japan took the opportunity to expand its sphere of influence in China and gain recognition as a great power in post-war geopolitics.
In WW1 Japan was a growing empire trying to take more land and get more resources to become a world power. Imperialism is the main reason they entered WW1.
If the states were each independent america would fail. The reason the states would fail is because each of them usually has a smaller population. Nor do they have much defense against each other.
