1914
_June 28_
Archduke Franz Ferdinand of Austria, heir to the Austro-Hungarian throne, is assassinated by a Serbian nationalist in Sarajevo, the provincial capital of Bosnia-Herzegovina (formally annexed by Austria-Hungary in 1908).
_July 28_
Austria-Hungary declares war on Serbia, after an ultimatum it gave to Serbia mobilized Russian forces–the first military action by a country outside the direct conflict.
_July 30_
Russian troops begin general mobilization.
_August 1_
Germany declares war on Russia.
_August 3_
Germany declares war on France.
_August 4_
Germany invades Belgium, prompting Britain to declare war on Germany.
1915
_February_
Germany begins unrestricted submarine warfare in the North Atlantic.
_April 25_
Allied forces land at Gallipoli, in an attempt to force Turkey out of the war.
_October_
Bulgaria joins the Central Powers.
1916
_February 21_
Germany begins the Battle of Verdun, which will last until December and claim the lives of more than a million men.
_July 1_
The Battle of the Somme begins. The British suffer 60,000 casualties on the first day, making it the bloodiest day in British military history.
_August 27_
Italy declares war on Germany.
_December 7_
The United States breaks off diplomatic relations with Germany.
1917
_January 31_
Germany announces the resumption of unrestricted submarine warfare.
_February 3_
The United States severs diplomatic relations with Germany.
_April 6_
The United States declares war on Germany.
_November 7_
The Bolshevik Revolution in Russia leads to the overthrow of the provisional government and the rise of the Bolshevik Party, led by Vladimir Lenin.
1918
_March 3_
The Treaty of Brest-Litovsk is signed between Russia and Germany, taking Russia out of the war.
_May 27_
The Second Battle of the Marne begins. The Allies launch a massive offensive that will eventually lead to the defeat of Germany.
_September 26_
The Meuse-Argonne Offensive begins. This is the largest battle in American military history, involving more than 1.2 million American troops.
_November 11_
The Armistice of 11 November is signed between the Allies and Germany, ending the war.
Casualties
The total number of casualties during World War I is estimated to be around 17 million, including both military and civilian deaths. Of these, around 10 million were military casualties, and 7 million were civilian casualties.
The war had a profound impact on the world, leading to the collapse of the Austro-Hungarian, German, Ottoman, and Russian empires. It also led to the rise of the United States as a global power, and the establishment of the League of Nations, an international organization aimed at preventing future wars.