Plans made without allowance for the intentions of the enemy are liable to miscarry. So it was to prove in 1916. - John Keegan

Lightning victories, experience tells, store up evil consequences, usually for the victors.

What was the issue was not the combatativeness of the British soldier but the still colonial outlook of their commanders, who expected decisive results for a comparatively small outlay of force and shrank from casualties. French generals, from a different tradition, expected large casualties, which their soldiers still seemed ready to suffer with patriotic fatalism. The British soldier, regular, Territorial, wartime volunteer, was learning a similar abnegation, while their leaders were coming to accept that operations in the new conditions of trench warfare could succeed only with the most methodical preparation. The qualities of dashing improvisation that had brought victory in mountain and desert for a hundred years would not serve in France.

The basic and stark fact, nevertheless, was that the conditions of warfare between 1914 and 1918 predisposed towards slaughter and that only an entirely different technology, one not available until a generation later, could have averted such an outcome.
The first day of the battle of the Somme, 1 July 1916, was to be an awful demonstration of that truth.