What single act caused the beginning of World War 1?

Name at birth: Franz Ferdinand Karl Ludwig Josef von Habsburg-Lothringen

Whatever else he may have done in life, Archduke Franz Ferdinand is known now as the man whose assassination touched off World War I. The nephew of the Hapsburg emperor Franz Josef, Ferdinand was first in line to the Austro-Hungarian throne when he visited Sarajevo in June of 1914. He and his wife Sophie were shot to death as they rode through the city in a motorcade on 28 June; the assassin was Gavrilo Princip, a member of the Serbian nationalist group known as the Black Hand. The shooting led to war between Austria and Serbia, which escalated into World War I.