There are a lot of reasons for the outbreak of the First World War. Plz give me exact reason which was caused first world war.

There are many reasons why the World went to war in 1914. These are some of the more important reasons.

Britain and Germany were competing to have the most powerful navy in the world. This caused tension in Europe.
The Scramble for Africa (when European countries had tried to get as many colonies as they could) had led to many small conflicts around the world. This meant that some of the major European powers were not very friendly towards each other.

The French were very distrustful of the Germans as a result of the war in 1870-1871 that the Germans had won.

The alliance systems set up to prevent war meant that any major political disputes would inevitably lead to a large rather than a small conflict.

Nationalism. People across the world were very eager to let the rest of the world how strong and important their country was. many people thought that their country was better than others: and thought that they'd be able to win a war very easily if there was one.

The Balkans region of Europe was claimed by both Russia and Austria (it was in general under Austrian control). This meant that the Russians and the Austrians would both be eager to enforce their authority over this region.

A desire for independence. many people in Europe lived in countries that were part of empires. They didn't all like being ruled by people with different languages and religions and this led to conflicts that could (and did) involve other nations.

The assassination of Archduke Franz Ferdinand in Sarajevo. He was the heir to the Austrian throne and was murdered by Serbian nationalists. This was the spark that caused war to be declared.