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Germany invades Belgium and France; stopped at the First Battle of the Marne. -
Archduke Franz Ferdinand is assassinated in Sarajevo, triggering tensions. -
Austria-Hungary declares war on Serbia — World War I begins. -
Germany uses poison gas for the first time at the Second Battle of Ypres. -
The RMS Lusitania is sunk by a German submarine, shocking the world. -
The Battle of Verdun becomes one of the bloodiest battles in history. -
Battle of the Somme — tanks are used in combat for the first time. -
The United States enters the war on the side of the Allies. -
The armistice is signed on November 11, ending fighting on the Western Front.