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Franz Ferdinand and his wife, Sophia on June 28, 1914.
The assassination of Archduke set of a chain reaction between alliances. And country's were forced to pick side. This caused the Balkan region to be the powder keg of World War I.
Austria
the assassination of archduke Franz Ferdinand of Austria caused different countries to to alliance with one and another so that if there was a war they would help each other, to protect the royals in the country. the more countries that joined each alliance began people to begin fighting for each country and this was when the out break of ww1 started.
It was the country of Serbia because there the assassination of archduke Franz Ferdinand took place, therefore igniting the war to start
Yugoslavia
Austria
The archduke Ferdinand was assassinated in Sarejevo in Bosnia/Herzegovina.
Austro-Hungary
The US didn't really know about the assassination, you see, after the assassination of Francis, the country declares war on them, if you learnes, Alliances was another effect that lead to ww1, as they country's started to fight their alliance helped them, and soon the world was in war
Serbia
Serbia