 "World War I")
Unleashed by an archduke's assassination, World War I gradually entangled more and more countries, killing millions of soldiers and civilians and touching multiple continents.
As the world marks 100 years since the fighting stopped, here is a look at some key moments in WWI:
1914
June 28: Serb teenager Gavrilo Princip kills Austrian Archduke Franz Ferdinand
July 28: Austria-Hungary declares war on Serbia
Aug. 1: Germany declares war on Russia
Aug. 3: Germany declares war on France
Aug. 4: Germany invades Belgium; Britain declares war on Germany
Aug. 23: Japan declares war on Germany
September: Battle of the Marne stops the German advance in France
Oct. 29: Ottoman Empire enters the war
November: Beginning of trench warfare
Dec. 25: Unofficial Christmas Truce
1915
February: German U-boat campaign marks first large use of submarines in warfare April: Allied troops land in Gallipoli , Turkey, a defining moment for Australian, New Zealand
April 22: First use of a chemical weapon, chlorine gas, near Ypres, Belgium
May 7: British ship Lusitania sunk by German U-boat
May 23: Italy enters war, against Austria-Hungary
October: Bulgaria joins war, on side of Central Powers
1916
Feb. 21: Battle of Verdun begins
March 9: Germany declares war on Portugal
July 1: Battle of the Somme begins, with first mass use of tanks
Aug. 27: Romania enters war, is invaded by Germany
Sept. 4: British take Dar es Salaam in German East Africa
October: Soldier Adolf Hitler wounded
Dec. 23: Allied forces defeat Turkish in Sinai Peninsula
1917
March: Baghdad falls to Anglo-Indian forces
April 6: The United States declares war on Germany
April: Battle for Vimy Ridge, a defining moment for Canada
July: Last Russian offensive ends in failure, as revolution nears; inconclusive Battle of Passchendaele in Belgium
Oct. 15: Spy Mata Hari executed by French firing squad
Oct. 26: Brazil declares war, joining Allied Powers
December: Battle of Jerusalem
1918
March 3: Treaty of Brest-Litovsk ends Russia's involvement in the war on the Eastern Front.
April 21: Legendary German fighter pilot known as the Red Baron shot down and killed near Amiens, France June: Battle of Belleau Wood, defining moment for US military
July 21: German submarine fires on Cape Cod, only attack on mainland US.
Sept. 26: Battle of the Meuse-Argonne begins
Oct. 30: Ottoman Empire signs armistice with Allies
Oct. 31: Dissolution of Austro-Hungarian Empire
Nov. 9: Germany's Kaiser Wilhelm II abdicates
