The outbreak of the First World War was precipitated by the assassination of the Austro-Hungarian Crown Prince Franz Ferdinand and his wife, by Serbian nationalist Gavrilo Princip. The assassination itself set in motion the network of military alliances that had been set up since the late nineteenth century.  

28 June, Archduke Franz Ferdinand, heir to the throne of the Austro-Hungarian Empire, and his wife Sophie are assassinated in Sarajevo, capital of Bosnia, by Gavrilo Princip, a member of the radical Serbian nationalist group, The Black Hand.
29 June, Secretary of the Austro-Hungarian Legation at Belgrade sends dispatch to Vienna accusing Serbian complicity in the assassination.
20 July, Austria-Hungary sends troops to the Serbian frontier.
25 July, Serbia orders mobilisation of troops.
Russia arranges for troops to be stationed on Russo-Austrian frontier.
28 July, Austria-Hungary declares war on Serbia.
29 July, Great Britain warns Germany that it cannot remain neutral.
Austrians bombard Serbian capital Belgrade.
German patrols cross the French border.
1 August, France orders the mobilisation of troops.
Germany declares war on Russia.
Italy announces neutrality.
Belgium announces neutrality.
3 August, Germany declares war on France.
Great Britain gives order for troops to mobilise.
4 August, Germany declares war on Belgium. 
United States declares neutrality.
Great Britain declares war on Austria-Hungary.
6 August, Royal Navy cruiser HMS Amphion is sunk by German mines in the North Sea, causing the death of 150 men and the first British casualties of war.
7 August
First members of the British Expeditionary Force (BEF) land in France.
20 August, Brussels is evacuated as Germans occupy the city.
23 August, Japan declares war on Germany.
Germany invades France
26 August, The Battle of Le Cateau, northern France - BEF suffers 7,812 casualties and is forced to retreat.
26 – 30 August, The Battle of Tannenberg (in north-eastern Poland) took place between the Russian Second Army and the German Eight Army. It resulted in the destruction of the Russian Second and First Army, rendering Russia ineffective on Germany’s eastern front until the spring of 1915. This left Germany free to move her troops to concentrate on the western front.
6 September, The Battle of Marne, Paris, France involved more than two million soldiers, lasting six days, in which   British and French troops succeed in halting the German advance on Paris at the cost of 13,000 British, 250,000 French and 250,000 German casualties. It put an end to Germany’s Schlieffen Plan, whereby the defeat of France via an invasion of Belgium was envisaged. German troops had succeeded in advancing within thirty miles of Paris. 600 Paris taxicabs were recruited to drive 6 000 French troops from the capital to the frontline. By 11 September, all the divisions of the German army deployed in the battle retreated. This was the first important victory for the Allied Forces.
16 October, The British Indian Expeditionary Force sails from Bombay to the Persian Gulf in preparation for the defence of Mesopotamia.
19 October, First Battle of Ypres, Belgium, begins on 19 October and ends on 22 November. This was the last battle fought between the Allied Forces and Germany in the race to the sea. The British Expeditionary Force under the command of Field Marshall John French reinforced French –Belgian Troops at Ypres. German troops had occupied Antwerp, and prepared to attack the Allied troops and take the city of Ypres. The Germans were beaten back and the onset of the Belgian winter forced them to withdraw. The German army lost 134 000 soldiers, the Belgian army lost 21 500 soldiers, while the BEF lost 7 900 men, 29 000 wounded and 17 800 missing.
29 October, Turkey enters the war on Germany side.
22 November, Trenches are established along the entire Western Front.
23 November, The British enter Basra, securing oil supplies in the Middle East needed to supply most of the Royal Navy.
8 December, The Battle of the Falkland Islands. A Royal Navy task force sinks three German cruisers that were victorious at the Battle of Coronel (Chile) in November. Only the SMS Dresden escapes.
16 December , The German First High Sea fleet bombards Hartlepool, Whitby and Scarborough, killing 137 civilians and proving that the British mainland is susceptible to attack.
25 December, Soldiers on both sides of the Western front declare a Christmas truce.

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