How did the assassination of archduke francis ferdinand and his wife sparked the outbreak of world war i?

As the Archduke was Austro-Hungarian, and after the attack, they assumed the Serbian government was responsible for the attack. At the time, Russia was a supporter of Serbia. The declaration of war on Bosnia was initially delayed because the Austro-Hungarians wanted to be reassured that they would have Germany's support if the Russians were to align themselves in the fight with Serbia. Except Russia was also allied with France. So on July 28 the declaration of war was released and any threads of peace were broken. In one week, Serbia, Russia, France, England and Belgium had gone up against Austria-Hungary and Germany and started WWI