People tend to know in simplistic terms why both World War I and World War II started, but often they do not seem to realise just how complex the situations were at those times. In the case of World War I most people know that assassination of Franz Ferdinand set of the war, but they don't realise all the factors that surround this. The Balance of Power System: Prior to the war the countries of Europe were arrayed in sophisticated systems of alliance. These systems were so closely balanced that the weakening of one country fatally weakened the entire alliance. This meant that if even once country was attacked then the two alliances effectively had to declare war on one another.