With all this remembrance of WW1, it's interesting to look at why it happened and what lessons we can learn for today.
I would disagree with No Right Turn that it was a "House of Saxe-Coburg-Gotha family squabble" - the various royal families had little in the way of personal disagreement and in Britain's case, relatively little influence on government.
The short version of the cause of WW1 is this:
Britain/France/Russia and Germany/Austro-Hungary/Turkey had formed competing alliances. They had various areas of conflict: notably the boundaries between France/Germany and the (still active) fault line between East and West in the Balkans.
These conflicts had been active for hundreds of years. Previous wars had been affairs of columns rather than fronts - two rival armies would march until they encountered each other and fought. This kind of war took time to start - troops had to be recruited, mobilised (on horseback) and brought to battle. The industrial revolution had facilitated a war of fronts - much larger numbers of troops could be mobilised by rail to enable rapid confrontation. Weapons had also evolved much more destructive power.
The key to success in modern warfare was seen as being to take the advantage and initiate a mobilisation plan for the enemy could resolve. For this reason, once one side began the march to war it could not be stopped. 1914 Europe also had few, if any, mechanisms to regulate international relations and prevent all out war.
After ww1 (and ww2) various such mechanisms *were* put in place, in the form of the League of Nations and, later and more succesfully, the UN. These were based on the idea that countries would obey various rules - most notably a self-imposed limit on their rights to initiate warfare.
These rules are now thought by some to be "inconvenient" or "quaint". The US/UK/Australian alliance has decided that it is reasonable to unilaterally attack countries that are considered objectionable, notwithstanding petty conditions of international legality. To me, the lessons of Gallipoli and the Somme are that we move down this path at our peril.