Causes of World War I

The First World War lasted from 1914 to 1918. It was caused by the buildup of tension between countries. The three long-term causes were the formation of empires, the accumulation of armies and weapons, and alliances. The trigger cause was the assassination of Archduke Franz Ferdinand.
One of the causes of World War I was that countries wanted to form empires. This is called "imperialism". From the late 1800s, the power of an empire is judged by its size. Britain had the largest empire, with lots of territories overseas, such as India. People would say "The sun never sets on the British Empire" because there were British holdings all over the world, so it was always day in some part of the empire. One way in which empires expanded their empire was during the Morocco Crisis, where the European superpowers colonised African nations obtained raw materials in order to fuel their economic growth. Furthermore, France was entitled to being the policeman over Morocco, but this escalated tensions with Germany to the extent that they sent the warship Panther. 
In order to conquer far-away places on different continents, imperial powers built large armies and navies and the cultures valued fighting. This was called "militarism". Countries spent large amounts of money in a short amount of time to build these armies. In the early 1900s, Britain's navy was the biggest. This made their traditional rival, Germany want to make a big navy too. When Britain launched a new kind of ship called the Dreadnought, Germany launched their own version of the Dreadnought to compete with Britain.
The third long-term cause was "alliances". The bigger countries of Europe formed two sides. They were called the "Triple Entente" and the "Triple Alliance." The Triple Entente included France, Great Britain, and Russia. The Triple Alliance was Germany, Austria-Hungary, and Italy. Great Britain, France, and Russia were worried about the growing strength of Germany, so they promised to help each other if any one of them was attacked. Germany objected to this arrangement because, with France and Great Britain on the west and Russia on the east, Germany would be surrounded. So Germany formed an alliance with Austria-Hungary and Italy.
The trigger cause of World War I was the assassination of Archduke Franz Ferdinand, who was next in line to be emperor of Austro-Hungary. He was killed in Sarajevo on June 28, 1914. He and his wife, Countess Sophie, both died. They were making a state visit to the capital of Bosnia. A Serbian secret society, the Black Hand, wanted Bosnia to break free from the Austro-Hungarian Empire and be a country by itself. They were supported by Russia. The Archduke and his wife arrived at 9:30 a.m. and got into an open-topped car at the train station. Members of the Black Hand had learned the route they would take to their destination. When the time came, one of them threw a hand grenade at the Archduke's car. It bounced off the back of the car, which drove on. It injured the driver and passengers in the car behind the Archduke instead. When the Archduke realized he was a target he immediately abandoned his planned trip and went to visit a hospital instead. However, the driver took a wrong turn up Franz Joseph Street and had to reverse. By chance, he slowed down in front of Gavrilo Princip, leader of the Black Hand. Seeing that this was his only chance to carry out his plan, he pulled his gun and fired, killing the Archduke, Sophie, who had been pregnant at the time. Gavrilo Princip was arrested and beaten up for this rebellious act. on the street in June 28, 1914. 
Austria-Hungary was outraged that the heir to their throne had been killed. They blamed Serbia and bombed the capital, Belgrade. The Serbians tried to surrender, but Austria-Hungary bombed them anyway. Serbia's ally, Russia, then helped to defend it from attack. Soon after, Germany, Austria-Hungary's ally, declared war on Russia as well as starting to move towards French territory, ready to invade, attacking Belgium as they progressed. The French soldiers started to get ready for a German invasion. When Germany declared war on France, then under the terms of the Triple Entente, Britain then ordered Germany to stop attacking Belgium, who was neutral during the war. When the Germans didn't retreat from Belgium, Britain declared war on Germany. Finally, Austria-Hungary declared war on Russia and WWI commenced. 
On Franz Joseph street there is a placard with what happened.