First, Serb student Gavrilo Princip shoots Austrian Archduke Franz Ferninand and his wife. As the Austro-Hungarians were unhappy with Serbia's response, they then declared war on Serbia. Due to her alliance, Russia mobilises in order to support Serbia. Because of the alliances Germany declares war on Russia. After that, Germany declared war on France. Britain declares war on Germany since they invaded Belgium using the Schlieffen Plan. To help Germany, Austro-Hungary declared war on Russia. And finally , France and Britain declare war on Austro-Hungary.