This article describes it in good detail: http://www.historyhome.co.uk/europe/causeww1.htm I will summarize it here. On June 28th 1914, in Sarajevo, Bosnia (Now Bosnia-Herzegovina), the heir to the Austro-Hungarian Empire, Archduke Franz Ferdinand was assasinated by a Serbian assasin, Gavrilo Princip. He, and his wife, were both killed. This caused the Austro-Hungarian Empire to impose an ultimatum to Serbia, also known as the "July Ultimatum". The Serbians accepted all clauses of said ultimatum, save for the 6th clause, which stated that Austrian police were to be allowed to operate inside Serbia. After that, Austria-Hungary declined the compromise that Serbia had made, and on July 28th, 1914, Austria-Hungary declared war on Serbia. After that, Russia declared itself on the side of Serbia, and Germany on the side of Austria-Hungary. As a result of Russia joining, France did as well. After France joined, the United Kingdom, and its commonwealth joined shortly after. I hope this answers all of your questions. If you have any further questions, let me know :)