The trigger point was the assassination of Austria's Archduke Ferdinand by a Serbian national, sparking off a war between Serbia and Austria. After that it was really just choosing up sides. There were of course the longstanding territorial disputes all over Europe and to a point , The Middle East. Plus the ever present economic questions about trade and successful economies vs faltering economies. Europeans have been fighting each other for thousands of years anyway and memories are forever. Plus the fact that most all European royalty were related to each other in one way or another, and family disputes go on forever. Thus, WW1.