Since you did not tell WHERE or WHEN you are asking about, no one can answer this question.
1 Britain joined because Germany was invadeing other countries fast and we did not want to be invaded 2 we warned the Germans not to invade Poland but they did so we declared war 3 it became a world war because we had an empire and with that empire came treaties. when we joined the rest of our empire had to help us. 4 Hitler was massacreing Jews whilst we fought to keep land and an empire 5 the empire crumbled and was no more after the war 6 Germany was told after the first world war not to have any relation with anything to do with an army and they did and there is probably more but they are the first ones off the top of my head lol :)