I do not believe that World War I was avoidable by the turn of the 20th century. The term "powder keg" is often used to describe the situation in Europe at the time and since Europeans had colonies all around the world, the explosion of this powder keg was bound to effect the entire world. While the trigger for the conflict for World War I was the assassination of Archduke Ferdinand of Austria, the real causes lie in the militaristic build-up of Europe, imperialistic policies that set European nations at each other's throats all over the world, secret alliances that required parties to attack if other parties were attacked, and a rampant nationalism that made each nation think they were better than the other.
If World War I had reached a better settlement and had followed the recommendations of Woodrow Wilson and had not required punitive reparations from Germany it is possible that World War II may never have been fought. However, once Hitler became an established power, I believe, there is no way we could have avoided war because no matter what Japan did, if Hitler had conquered all of Europe, it would have had dire consequences for the United States, because he would have not stopped with Europe. Further, I believe that Hitler's racist evil needed to be stopped or whole peoples would have been wiped off the face of the earth. There are times when war is just and righteous and WWII was one of those times.