I suggest that the reason people both at the time and now struggle to make sense of the vast human toll of the Great War is because the principle it was fought to defend seems so insubstantial. In the end, for the British Empire at least, it was a war for the rule of law.
The United Kingdom's stated reason for entering the war was the defence of Belgian neutrality. In 1839 Germany signed the Treaty of London which created the Kingdom of Belgium, with that Kingdom recognised as remaining a neutral power. It was that promise of neutrality which the German Empire intended to violate by executing the Schlieffen Plan in order to inflict a rapid defeat on France.
Treaties - promises between nations - are a form of law: ultimately they are a promise by a sovereign to behave in a particular way, in the same way that an Act of Parliament is a promise that some behaviour (like murder) will not be countenanced. By making war on Belgium, the German Empire repudiated its sovereign promise. Britain's acceptance of this as a cause for war declared that it was willing to shed blood to maintain the rule of international law. Its willingness to endure horrendous bloodshed in this cause was a demonstration that the rule would not be violated with impunity (1).
I think that the sacrifice of the Great War seems meaningless today is because the principle that sovereignty is not unlimited is now taken for granted. The sovereign power of making war (or of "making die") (2) that President Hussein sought to exercise with the annexation of Kuwait in 1990 prompted Security Council resolution 678 and ultimately resulted in the Persian Gulf War. At present a long-running dispute between Nicaragua and Costa Rica over the location of their border has lead not to a call to arms or a gallant defence of the Fatherland, but to proceedings in the International Court of Justice.
It is for this reason that I find the hostility to international courts of William Safire (3) and Ted Cruz (4) to be somewhat concerning. These courts are a sign that law is accepted to exist at an international level, and that state sovereignty ought conform itself to it - precisely the principle for which the British Empire went to war. Courts like the International Court of Justice - slow, procedure-heavy, grindingly technical, unexciting - are the lasting monument to the courage and suffering of the British- and Mediterranean Expeditionary Forces. I can think of few greater gifts to the people of the world.
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(1) An interesting 'what if' is whether Britain would still have entered the war against Germany. Personally I think it would: the potential gains in the form of not allowing a single power to dominate Europe, and the promise of territorial gains for New Zealand, Australia and South Africa, would ultimately have been too great a temptation for the Empire to remain aloof.
(2) Sophie Wahnich, In Defence of the Terror (trans. D. Fernbach) (2012), at 58-59.
(3) William Safire, 'The Purloined Treaty', New York Times (9 April 2001) at np.
(4) Ted Cruz, 'SCOTUS rejects authority of World Court', Human Events (1 April 2008) at 1.
