The European balance of power broke down was an effect following German unification is correct answer due to fact that Germany (except Austria) was united by militarist state of Prussia with powerful army which definitely change balance on the European scene especially that formal unification took place just after end of Franco-Prussian war which humiliated France.
Austria declared war on Serbia in a calculated response to the murder of Archduke Franz Ferdinand. There was much blame on both sides as to why war came about and what the true cause was, but the murder was the reason used.
