What was the cause of the First World War?

The best way to go about answering a causation question such as this is to break your answer down into individual causes and then use linkage sentences and the conclusion to draw the causes together. It may also be relevant to draw in historiographical arguments when necessary to add depth to your answer and demonstrate a nuanced response to the question.
Although in the immediate post-war period the historical narrative tended to place the blame for the outbreak of world war one with Germany, the causes of the war are much more complex and no single nation was to blame. Similarly, the assassination of Archduke Franz Ferdinand in June 1918 acted as a trigger for the outbreak of the war, yet by itself did not cause the war as tension had been increasing in Europe for decades. The main causes of the first world world war were competing aspirations of European states, the unravelling of the Bismark system by Kaiser Wilhelm of Germany, a secretive alliance system, Serbian nationalism and conflicts in Morocco and the Balkans. These causes were all interlinked and affected each other as they played out, which explains why the assassination of Archduke Franz Ferdinand acted as a catalyst for a global war.