Hello again (I'm afraid I can only answer this question today since I'm busy) the Assassination of the Archduke Franz Ferdinand of Austria caused WW1 and a chain effect (it also rose the tensions between Austria and Serbia since the killer was from Serbia). This caused Austria-Hungary to declare war on Serbia and eventually everyone else joined (tensions were already high back then) in for they were looking for a excuse to start a war. Some of the countries include Germany, France, Russia and Belgium.
Hopefully this helped! :,)