How did one bullet lead to world war 1?

Answer 1

The bullet was what killed arch duke Franz Ferdinand, heir to the Austrohungarian throne. He was killed by a group called the Black Hand, but Austria blamed Serbia and declared war on them. Russia had sworn to help Serbia if they were attacked, and so stepped in to help. Germany, an ally of Austria, declared war on Russia after hearing about their involvement with Serbia. Britain began mobilising its war ships, and Germany declared war on France - a Russian ally - in an attempt to avoid a battle on two fronts. Britain and Belgium declared war on Germany after German troops marched through Belgium. The bullet did not kill all of the people, but its effects did.

Answer 2

It's just not that simple. It's easy to think that if Archduke Ferdinand had not been shot WW1 would not have started but that would ignore many other factors, such as the volatility of the region going back many (many) years.