What was the "spark" that set off World War I?

The first assassination attempt failed due to the quick reaction of the Austrian heir-apparent. Out of the corner of his eye, Franz Ferdinand saw something black flying towards him and raised his hand in a protective gesture, thus knocking the hand grenade out of the car. It landed on the car behind him, injuring two of that car's passengers. The perpetrator tried to commit suicide by swallowing a cyanide pill, but as the pill was old, the poison failed to work and only made him sick. He was stopped by people on the street and arrested. The archduke's procession sped quickly to City Hall. There, it was decided that they would take a different route to the grounds of the military parade.

Not long after starting off for the grounds, the driver of one of the cars leading the procession noticed that they were going the wrong way. The cars had to slowly turn back. Meanwhile, a second conspirator had taken up position. He seized his opportunity, and fired his pistol twice into the car. Franz Ferdinand was shot in the neck; Sophie in the abdomen. Their attacker also swallowed a cyanide pill, which also failed to work. While the second attacker was held by passersby, Franz Ferdinand and his wife Sophie died in their car from their injuries.

At the trial of the two attackers, they admitted to being followers of the "Pan-Slavist" movement. With the support of Russia, this organization aimed for national unity for all Slavic people. As many Slavs lived in the Austro-Hungarian Dual Monarchy, the heir to the Austrian throne had long been identified as a potential assassination victim.