Liddell Hart's conclusion, basically, was that the heavy casualties suffered in World War I were due primarily to the incompetence of the officers in charge.
Most of the lessons needed to conduct this war effectively were revealed in the American Civil War, which was the first major industrial war (machine guns, trains to move troops, ironclad ship, but more importantly, a war where the industrial base of a nation was recognized as a proper military target). Tactics and strategies were developed in America that were suitable to mechanized warfare. These lessons were ignored by the European officers, who looked back to the Napoleonic wars and Carl Von Clausewitz for guidance. The Americans, when they arrived in 1917, were not blind to the lessons of their own history, and consequently conducted themselves a little more sensibly.
Here is a list of percentage of soldiers killed, for major participants of the war:
Romania 44.76% France 16.36% Germany 16.12% Austria-Hungary 15.38% Russia 14.17% Italy 11.58% Turkey 11.40% Great Britain 10.20% US 2.89%