Unleashed by an archduke's assassination, World War I gradually entangled more and more countries, killing scores of soldiers and civilians before touching multiple continents.
As the world marks 100 years since the fighting stopped, here is a look at some key moments in WWI:
1914
THIRTY EIGHT OF ONE HUNDRED PHOTOS WORLD WAR ONE CENTENARY TIMELINE-In this 1915 file photo, Turkish soldiers form a queue to get water from a fountain, behind their lines in Gallipoli, Turkey. (AP Photo, File)
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
SEVENTY OF ONE HUNDRED PHOTOS WORLD WAR ONE CENTENARY TIMELINE-In this undated file photo, dancer "Mata Hari", who was born Margaretha Geertruida Zelle poses in an unknown location. Zelle was executed October 15, 1917 during World War One for spying for Germany. (AP Photo, File)
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
THIRTY SEVEN OF ONE HUNDRED PHOTOS WORLD WAR ONE CENTENARY TIMELINE - In this undated file photo, an unidentified soldier, wearing a gas mask around his neck, bangs on a frying pan as a gas warning on the field near Reims, France. (AP Photo, File)
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
FILE - In this undated file photo American World War I soldiers wave their helmets after the Nov. 11, 1918 Armistice was signed in France. Hundreds of troops died on the final morning of World War I _ even after an armistice was reached and before it came into force. Death at literally the 11th hour highlighted the futility of a conflict that had become even more incomprehensible in four years of battle. (AP Photo, File)
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: United States declares war on Germany
April: Battle for Vimy Ridge , 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 U.S. military
July 21: German submarine fires on Cape Cod, only attack on mainland U.S.
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
Nov. 11: Germany signs armistice ending the war
For more information on World War I, go to The Associated Press' WWI hub: https://www.apnews.com/WorldWarI
World War I: An AP Centennial Commemorative Edition. Available now exclusively at Amazon: https://amzn.to/2JGrx5U