PARIS — 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

In this June 28, 1914 photo, the Archduke of Austria Franz Ferdinand, center right, and his wife Sophie, center left, walk to their a car in Sarajevo. This photo was taken minutes before the assassination of the Archduke and his wife, an event which set off a chain reaction of events which would eventually lead to World War I. (AP)
In this June 28, 1914 photo, the Archduke of Austria Franz Ferdinand, center right, and his wife Sophie, center left, walk to their a car in Sarajevo. This photo was taken minutes before the assassination of the Archduke and his wife, an event which set off a chain reaction of events which would eventually lead to World War I. (AP)

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

The U Boat 139 sank the Lusitania in an unknown location. The British liner sank on May 7, 1915, off the southern coast of Ireland. About 1,150 men, women and children, perished, 114 of which were Americans. Among those Americans who perished were Charles Frohman, Alfred G. Vanderbilt and Elbert Hubbard. (AP)
The U Boat 139 sank the Lusitania in an unknown location. The British liner sank on May 7, 1915, off the southern coast of Ireland. About 1,150 men, women and children, perished, 114 of which were Americans. Among those Americans who perished were Charles Frohman, Alfred G. Vanderbilt and Elbert Hubbard. (AP)

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

WWI death toll compared with other modern conflicts
WWI death toll compared with other modern conflicts

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