Princip was born in Obljaj a poor area of Bosnia. As a young man he joined a small group who wanted the Slavs in the south of Austria-Hungary to join with Serbia in a new country. They called themselves "Young Bosnia". A group of powerful people in Serbia decided to help them. Those powerful people were called the Black Hand. They gave 'Young Bosnia' guns and small bombs.
When they heard that Franz Ferdinand was coming to Sarajevo, six of them took the bombs and guns and waited for Ferdinand to drive past. They were going to kill Ferdinand when he drove past. Every one of them failed. After they all failed, Princip went to a cafe to buy a sandwich.
When Princip came out of the cafe, he saw Ferdinand in his car. The driver of the car had driven his car the wrong way and he stalled the car while trying to turn. Princip walked up and shot Ferdinand. He wanted to shoot the general who was also sitting in the car, but Ferdinand's wife Sophie threw her arms around Ferdinand. She was in the way, and Princip shot her instead of the general.
At his trial, Princip said he did it because he loved his people. The Austrians thought it was a very bad crime, but they could not kill him because he was only 19 years old. The law in Austria-Hungary said only people older than 20 could be killed, even when the crime was as bad as killing royalty like Ferdinand. They said Princip must stay in prison for 20 years. During his time in the prison, one of his arms was amputated. He died in prison of tuberculosis and blood loss in 1918.
Because this crime happened, Austria-Hungary gave Serbia a list of things that Serbia had to do, later called the "July Ultimatum", to stop things like this from happening any more. Serbia could not do all the things on the list fast enough, because the Austrians really wanted war with Serbia. Their German allies also wanted a big war. This led to many of the European countries to start fighting.