There is a significant difference between writing the history of a subject some of whose sources are still living and breathing, and that of a subject whose sources are only documentary. In the past decade or two the First World War has undergone a transition from one to the other. A few centenarians can remember soldiers marching through the streets on the way to the front, or home in parades to mark what proved to be a hollow victory. Otherwise, the written word in memorandums, letters, diaries, despatches and memoirs forms the basis of our understanding of the period, together with extensive recordings made of old soldiers, sailors and airmen from the 1950s onwards.

That treasure trove of eyewitness accounts – what earlier generations routinely heard while sitting on the knee of their fathers and grandfathers – has enormous value: but old men forget. History is riddled with examples of memory warping reality; we know it from our own experience. Also, the old men, heroes though many of them were, saw only their own microcosm of war. The value of their experience is not to be diminished by the fact that the bigger picture, inevitably, escaped them. The contemporaneous record, if it exists, may well be superior, provided we have the judgement to see when for propagandistic reasons it has been doctored or distorted, or censored. While the last Tommies were still alive a natural deference to their experience shone through much of the writing about the period. Now no one is alive who served in the trenches or on a dreadnought, and the reliance is entirely upon documents, there can be, paradoxically, far more rigour in the analysis, as sources are tested against each other, and the unreliability of active memory ceases to intrude.

In looking at the British historiography of the First World War one starts with writing that is absolutely contemporaneous, and ends with that from this first era beyond living memory. Aside from history written as history there is also history written as literature – Sassoon, Graves, Blunden and so on. One also becomes conscious of the compartments into which the story of the war must be placed. Few historians have the range of specialisms needed, at least in the depth to which each is required, to tell the whole story, and the few who have tried usually have failed.

First, an understanding of the history of power, international relations since (at least) the Congress of Berlin and of European diplomacy is required to illuminate the catastrophe of August 1914. One also requires a knowledge of the political heritage and divisions in certain countries that played a leading role in the drama: Austria-Hungary and its tensions with Serbia before and after the annexation of Bosnia; the question of Belgian neutrality; the history of the rivalry between King Edward VII and his nephew Kaiser Wilhelm II, which helped create sufficient distrust in the kaiser of the British that it coloured his feelings towards this country after Edward had died and his far less provocative son George V succeeded to the throne; the feelings in France since the war with Prussia of 1870-71; and the growing chaos in Russia and its relations with states in eastern Europe.

Second, one needs the skills of the advanced military historian not simply to outline strategy and tactics after war breaks out, and to recount the movements of troops and the joining of battle, but also to link these with the political direction (or, sometimes, lack of it) of the chancelleries of Europe. Here the documentary evidence takes on a new importance, following the chain of events from politician to general to the man in the trench. One must understand the democratic pressures on politicians – in those countries with a reasonable semblance of democracy – to advance the interests of their country through warfare: understanding why, for instance, such a deluge of young men joined Kitchener’s army in the first three months of the war, and the cultural pressure not simply on them to do so, but on the government to make the most of their service.