PARIS, France (AP) — 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.

August 1: Germany declares war on Russia.

August 3: Germany declares war on France.

August 4: Germany invades Belgium; Britain declares war on Germany.

August 23: Japan declares war on Germany.

September: Battle of the Marne stops the German advance in France.

October 29: Ottoman Empire enters the war.

November: Beginning of trench warfare.

December 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 Australia, 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

February 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.

August 27: Romania enters war, is invaded by Germany.

September 4: British take Dar es Salaam in German East Africa.

October: Soldier Adolf Hitler wounded.

December 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.

October 15: Spy Mata Hari executed by French firing squad.

October 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.

September 26: Battle of the Meuse-Argonne begins.

October 30: Ottoman Empire signs armistice with Allies.

October 31: Dissolution of Austro-Hungarian Empire.

November 9: Germany’s Kaiser Wilhelm II abdicates.

November 11: Germany signs armistice ending the war.