How World War I became one of the deadliest war in history

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: 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 US military

July 21: German submarine fires on Cape Cod, only attack 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

Nov. 11: Germany signs armistice ending the war

(With inputs from agencies.)