Why did World War 1 break out?

Garvillo Princip (The black hand gang) assassinated Franz Ferdinand.

Timeline:

• 28 June 1914: Archduke Franz Ferdinand (heir to the Austrian throne) and his wife, Sophie, were assassinated by Princip, a Serbian in Sarajevo.
• 28 July 1914: Austria blame the assassination on the Serbian government and uses it as an excuse to attack Serbia.
• 29 July 1914: Russia has promised to protect Serbia against any Austrian attack and begins to prepare its army.
• 1 August 1914: Germany hears about the Russian preparations and declares war on Russia.
• 2 August 1914: Britain mobilises its fleet of warships.
• 3 August 1914: Germany declares war on France. This is either because Germany is worried about a French attack, or because the Schlieffen Plan says France has to be attacked before Russia.
• 4 August 1914: Britain declares war on Germany.