Answer:
The least complex answer is that the quick reason was the death of Franz Ferdinand, the archduke of Austria-Hungary. His passing because of Gavrilo Princip – a Serbian patriot with connections to the undercover military gathering known as the Black Hand – moved the real European military forces towards war.
June 28, 1914: Archduke Franz Ferdinand, beneficiary to the Austrian position of royalty, and his significant other, visit Sarajevo in Bosnia.
July 23: Austria-Hungary, with the sponsorship of Germany, conveys a final offer to Serbia.
July 25: Austria-Hungary cuts discretionary ties with Serbia and starts to activate.