As far as Britain and her dominions were concerned the Western Front was the most costly event of modern history, and we remain touched by its long cold shadow.
Two good lectures on this subject on Youtube:
The image of the British army of 1914-18 as being inept, ‘lions led by donkeys’, is highly misleading. In fact, against a background of revolutionary changes in the nature of war, the British army underwent a bloody learning curve and emerged as a formidable force. In 1918 this much-maligned army won the greatest series of victories in British military history.