Although the assassination of Austrian Archduke Franz Ferdinand is cited as the main cause that thrust the world into war, many causes and events led up to World War I. Some of the secondary causes include the mutual defense alliances in place at the time, nationalism, imperialism and militarism.
Archduke Franz Ferdinand was assassinated in Sarajevo, a city in Herzegovina and Bosnia. Austria-Hungary had seized control of this territory to claim its resources, a policy called imperialism. The assassin was a Serbian nationalist who thought that the territory should belong to Serbia. This extreme belief that a country must demonstrate its power and authority is emblematic of the nationalism that helped fuel World War I.
As a result of the assassination, Austria-Hungary declared war on Serbia. Russia declared war on Austria-Hungary because it had a mutual defense agreement with Serbia that stated the allies of a country would fight in defense of it. Germany allied itself with Austria-Hungary. France allied itself with Russia, which resulted in an alliance with Serbia. Because France entered the war and was attacked by Germany, Britain and Belgium entered on France's behalf, followed by Japan on Britain's behalf. Eventually Italy and the United States joined forces against Germany and Austria-Hungary.