The assassination of Archduke Franz Ferdinand is widely believed to have triggered the events leading to the First World War, prompting a web of treaties and alliances that led to war on an unprecedented scale.
Here is the countdown to the Great War:
June 28, 1914
Archduke Franz Ferdinand, heir to the throne of the Austro-Hungarian Empire, and his wife Sophie are shot dead in Sarajevo.
The assassinations are the work of Serbian nationalists who want to bring the empire's Slav territory into a greater Serbia.
July 23, 1914
Austria-Hungary's reaction to the killings comes three weeks later, when it sends an ultimatum to Serbia.
It gives Serbia 48 hours to comply with its strict terms.
July 24, 1914
Germany declares its support for Austria.
British Foreign Secretary Sir Edward Grey asks that Germany, France, Italy and Britain should "act together for the sake of peace simultaneously".
July 25, 1914
Serbia orders its troops to mobilise while Russia, bound by treaty to Serbia, arranges for soldiers to be stationed on the Austrian frontier.
July 28, 1914
Austria-Hungary declares war on Serbia.
July 29, 1914
Britain warns Germany that it cannot remain neutral in the conflict.
On the same day, Austria bombards the Serbian capital of Belgrade.
July 31, 1914
Russian general mobilisation is ordered. Austria also mobilises its troops
Germany prepares for war and sends an ultimatum to Russia demanding that military preparations are stopped within 12 hours.
Britain asks France and Germany to declare their support for the ongoing neutrality of Belgium - but while France agrees, Germany does not respond.
Germany asks France whether it would stay neutral in case of a war between Germany and Russia.
August 1, 1914
Germany declares war on Russia.
French mobilisation is also ordered.
August 3, 1914
Germany declares war on France.
Britain gives the order for its troops to mobilise.
Germany tells Belgium that it will "treat her as an enemy" if the country does not allow free passage of German troops across Belgium land to fight the French.
Britain gives Austria-Hungary an ultimatum to stand down from hostilities.
August 4, 1914
Britain, having failed to receive notice from Germany assuring the neutrality of Belgium, and citing a "moral obligation" to defend France and Belgium, declares war on Germany.