July 28 - Austria-Hungary declares war on Serbia
* The assassination of Archduke Franz Ferdinand of Austria, heir to the Austro-Hungarian throne, by a Serbian nationalist on June 28, 1914, is considered the spark that ignited World War I.
* Austria-Hungary's declaration of war follows an ultimatum to Serbia that Serbia considered unacceptable.
August 1 - Germany declares war on Russia
* Germany, as Austria-Hungary's ally, declares war on Russia in support of Austria-Hungary's actions against Serbia.
August 3 - Germany declares war on France
* Germany declares war on France, which had been allied with Russia since 1894.
August 4 - Germany invades Belgium
* Germany invades Belgium, which had been guaranteed neutrality by Britain, France, and Germany in the 1839 Treaty of London.
August 4 - Britain declares war on Germany
* Britain declares war on Germany in response to Germany's invasion of Belgium.
1915
May 23 - Italy declares war on Austria-Hungary
* Italy, a former member of the Triple Alliance with Germany and Austria-Hungary, declares war on Austria-Hungary, siding with the Allied Powers (Britain, France, Russia, and Serbia).
October 14 - Bulgaria declares war on Serbia
* Bulgaria, allied with Germany and Austria-Hungary, declares war on Serbia.
1916
August 27 - Romania declares war on Austria-Hungary
* Romania, hoping to gain territory from Austria-Hungary, declares war on Austria-Hungary.
1917
April 6 - United States declares war on Germany
* The United States, which had been a neutral country throughout the early years of the war, declares war on Germany in response to Germany's unrestricted submarine warfare campaign against Allied shipping.
November 7 - Russia withdraws from the war
* The Russian Revolution leads to the Bolsheviks seizing power and the new government signing an armistice with Germany on December 15.
1918
May 29 - Armenia declares war on Turkey
* Armenia, which had been part of the Russian Empire, declares war on Turkey in the midst of the Armenian genocide.
July 19 - Czechoslovakia declares war on Germany
* Czechoslovakia, a new country formed after the collapse of Austria-Hungary, declares war on Germany.
October 28 - Austria-Hungary surrenders to the Allies
* Austria-Hungary signs an armistice with the Allies, effectively ending its participation in the war.
November 3 - Bulgaria surrenders to the Allies
* Bulgaria signs an armistice with the Allies.
November 11 - Germany surrenders to the Allies
* Germany signs an armistice with the Allies, officially ending World War I.
November 22 - Turkey surrenders to the Allies
* Turkey signs an armistice with the Allies, officially ending its participation in the war.