If I understand correctly, you need an example of a short term cause of WW1 (there was more than one of course!)
The most direct cause of WW1 was the assassination of the Austrian Crown Prince Francis Ferdinand in Sarajevo in 1918 by Serbian Rebels. Serbia was supported by Russia, so this led to a conflict between Austria and Russia.
Another direct cause for WW1 was the ultimatum by Austria to the Serbs which followed this assassination, which included some demands that were hard for Serbia to accept, and which included the intervention of Austria into Serbian internal affairs (such as into Serbian police work).
These (and some other) short-term causes, together with the long term causes, such as imperialism lead to the onset of WW1.
