Category : Causes of WWI
The assassination of Archduke France Ferdinand of Austria-Hungary was definitely the most immediate cause of WWI. Franz Ferdinand and his pregnant wife Sophia were assassinated by a young man named Gavrilo Princip who was in a Serbian terrorist group, The Black Hand. He was murdered when he was visiting Sarajevo, the capital of Bosnia and Herzegovnia (Assassination of Franz Ferdinand, historyonthenet).
After he was killed, Austria-Hungary send an ultimatum to Serbia to ask for compensation, if not Austria-Hungary will immediately start war with Serbia. However, Serbia didn’t agree with the ultimatum so in July 1914 Austria-Hungary declared war on Serbia.
Since Russia was allied to Serbia, Russia helped Serbia to mobilize Austria-Hungary’s border. By the time Austria-Hungary was in the alliance with Germany, so Germany soon declared war on Russia to help Austria-Hungary. Since then the war began to expand because both of the two allies were involved.