1. June 28th, 1914: Archduke Franz Ferdinand of Austria and his wife Sophie were assassinated by 19-year-old Gavrilo Princip, a Bosnian Serb nationalist. The archduke traveled to Sarajevo to inspect the imperial armed forces in Bosnia and Herzegovina. These forces annexed to Austria-Hungary in 1908 and that movement made Serbian nationalists extremely upset, because they firmly believed those territories should be part of Serbia.
2. Austria-Hungary blamed the Serbian government for the attack. As Russia supported Serbia, Austria asked Germany for help against Russia and its allies, including France and Great Britain.
3. On July 28, 1914, Austria-Hungary declared war on Serbia.