Why did japan enter world war i on the allied forces' side?

a. it had a previous treaty of alliance established with england.
b. it was angered when germany began sinking its ships in neutral water.
c. it disagreed with the german action of invading neutral belgium.
d. it was an ally of austria and joined after the archduke was assassinated.

Japan entered World War I on the Allied Forces' side because it had a previous treaty of alliance established with England. The correct option among all the options that are given in the question is the first option or option "A". Another reason might have been that Japan thought that since the Germans were busy in Europe, they could grab their colonies easily.

They had to go back and forth to the atlantic ocean and the pacific ocean to build the panama canal thats the best i can answer