I agree with Sir Max Hastings. He sums up what is undoubtedly a complex issue that academics discuss in depth on a regular basis. In fact it is amazing that 10 historians have come up with 10 answers; normally 10 historians would come up with 12 answers! But there again perhaps they have here!
It was a war that could have been avoided, that had roots going back to 1870 and the Franco-Prussian War. Alliances, allegencies, treaties, and general bad politics amoungst the European powers, then an arms race between Germany and Britain, with stubbornness/stupidity that finally led to German mobilisation that once started could not be stopped.
Britain certainly did not need to involve itself in a land war with it's small standing army. The Royal Navy, supremely powerful and designed to fight two nations navies at once, should have mounted a great blockade of ports supplying the German war machine. Later, around 1917, if Lord Kitchener had got his way, his modern army of by then 6 million men, fresh and well trained, could have been finally pushed into France for a mass offensive against a German army that by then would have been war weary. But instead a "cavalry" type attitude by General Haig and his key officers that led to the disastrous attacks against well dug in enemy. It was crazy, and some would say criminal. It should never have transpired, nor should the war itself! It was power play and very bad Imperial politics.