The First World War was believed to be caused by a number of reasons. Historians had many different viewpoints on the issue. Some believed that the war was caused from pre-existing tensions such as the alliance system whilst others thought that the events of July 1914 were the main problem and drove the key decision makers.
The Alliance system was formed by the great powers of Europe. The formation of the Triple Alliance began when the German Chancellor, Otto van Bismarck was conscious of the need of allies for the new young Germany. He was threatened by Russia and France who would want revenge after recovering from the Franco-Prussia war, 1870. Therefore Bismarck created a Dual Alliance with Austria-Hungary for defensive reasons and to isolate France. However by 1914 the original intentions of the alliances were different to their outcomes. Germany issued the Blank Cheque in July 1914 to Austria, saying they would help Austria-Hungary if they wanted to attack Serbia as well as during the Balkan wars.
This was a change in nature of the alliances from defensive to offensive which further increased European tensions.
In 1892 Russia joined a defensive alliance with France to keep the balance of power, however once France joined the alliance because of fears of Grmany and mutual distrust of Great Britain. This entente soon became aggressive when Russia began its great military program to increase its army with French help, making world war more likely through a chain of events.
However some historians believe that the Alliance system wasn't important in stimulating the build-up of European tensions. A. J. P. Taylor claimed that the pre-1914 alliances were very fragile and unstable which cannot be seen as a major cause of war. For example the formal alliance that Italy had with Germany and Austria-Hungary...