What were these forces and how did they contribute to the outbreak of WWI?

World War I is different from most other wars in that it was not a direct result of aggression by one entity towards another, notwithstanding the assassination of Austrian Archduke Franz Ferdinand by Serbian elements. Instead, it was a consequence of a number of forces amidst a disruption of the tenuous balance of power in Europe. What were these forces and how did they contribute to the outbreak of WWI?

300 word count. Minimum of two resources.