How did the japanese get involved in world war 2?

They had an alliance with Great Britain witch put them in a state of obligation to help GB or any other allied country in a time of war or other need ultimately making them an allied country.). visit this link it will help you alot http://en.wikipedia.org/wiki/Japan_during_World_War_I After signing the "Anglo-Japanese alliance" with Great Britain Japan helped the allied cause by attacking and taking over the German colonial port of Tsingtao in China.