The assassination of Archduke Franz Ferdinand of Austria-Hungary. Austria-Hungary's response, blaming Serbia, set off a chain of alliances and declarations of war

June 28, 1914:

Austria-Hungary officially declares war on Serbia

July 28, 1914

Germany declares war on Russia, France, and Belgium. Britain declares war on Germany in response to its invasion of Belgium, a neutral country. Soon after, Austria-Hungary declares war on Russia. France and Britain declare war on Austria-Hungary, and Japan joins the Allies by declaring war on Germany."

August 1914

Germany's advance through Belgium is halted at the First Battle of the Marne, leading to a stalemate and the beginning of trench warfare

September 4, 1914

First battle of Ypres

October 31, 1914

Britain and France declare war on the Ottoman empire

November 5, 1914

German zeppelin raids on Britain begin

January 10, 1915

Germany begins unrestricted submarine warfare, targeting all ships, including civilian vessels

February 4, 1915

Second battle of Ypres, first use of chemical warfare

April 22 - May 25, 1915

ANZACs arrive at Gallipoli

April 25, 1915

Italy declares war on Austria-Hungary, formally joining the Allies

October 6, 1915

Austro-Hungarian and German forces invade Serbia in a coordinated campaign but are eventually forced to retreat

October 6, 1915

November 11, 1918