Pansyface, simple 1) Austrian Empire rules the Balkans badly. 2) Balkan separatists decide it must end. 3) Austria declares war on Serbia. 4) Russia comes in on Serbia's side. 5) Germany comes to Austria's aid. Russia has a pact with France. 6) Germany decides to knock out France with one blow invading through Belgium. 7) Britain has pact with low countries and declares war on 4th August 1914.
One unconnected fact has a domino effect on Europe which ended in 18,000,000 killed world wide. Our politicians have this idea that a good war will enhance their status, the very young men and women who fight those political wars have a word for those people not printable here and I still use it.