Franz and Sophie decided to visit the bystanders who were wounded, however the driver took a wrong turn directly into Franzjosefstrasse. This was the perfect opportunity for Gavrilo Princip to shoot them with his gun while he was standing there to buy his lunch.
This assassination set off a chain of events which descended into war.
Austria declared war on Serbia on July 28th, 1914. Russia supported Serbia and went to war to protect them. The WWI was officialy declared, Germany, Austria-Hungary, Bulgaria and the Ottoman Empire against Great Britain, France, Russia, Italy, Romania, Japan and the USA. The WWI ended in Nov 11, 1918