The spark that caused WWI was the assassination of a prince in Austria-Hungary (AH for short) because Siberia was upset that they weren't able to get a certain city from them. That was the spark. What caused the explosion was the alliances between all of the countries. Siberia didn't have a big enough military to fight AH after they declared war on them so they asked Russia for help. Then AH asked for Germany's help. Then Russia and Siberia asked for Frances help and so on. This small event combined with these alliances caused a big blow-out war that was arguably fueled by nationalism by the way. So that was the spark that caused the explosion
