The debate on the origins of the First World War started almost as soon as the conflict broke out in August 1914. Both sides quickly published collections of diplomatic correspondence purporting to show that the blame for the war lay elsewhere. The British Blue Book, for example, argued that Britain had been forced to intervene against aggressive 'Prussian' militarism (and in defence of small nations such as Belgium and Serbia). Germany's White Book aimed to show that it was fighting a defensive war against Russia (and in defence of its wronged ally, Austria-Hungary).

This debate, which bubbled away under the surface during the conflict, was re-ignited in June 1919 by the Treaty of Versailles, which (in Article 231) laid the blame for the origins of the 'Great War' firmly at the door of the Central Powers. In the nascent German republic, founded in the wake of defeat in November 1918, the 'war guilt clause' became a detested symbol of German enslavement. The Weimar government formed a special branch of its Foreign Office dedicated to rebutting the 'lie' of German responsibility for the First World War.

Germany's case for the defence rested on two main ideas: first, that the Serbian government was complicit in the murder of the heir to the Habsburg throne, Archduke Franz Ferdinand, in Sarajevo on 28 June 1914; and, second, that a general European war was ultimately provoked by Russia's unswerving support for Serbia and its subsequent decision to order a general mobilisation on 30 July.

Neither argument was wholly invalid. The Serbian military leadership, led by its intelligence chief Dragutin Dimitrijevic, did indeed train terrorists such as Franz Ferdinand's assassin, Gavrilo Princip. Equally, the firm pro-Serb stance adopted by the Russian foreign minister, Serge Sazonov, during the July crisis, coupled with his unwillingness to negotiate with Berlin or Vienna, did little to ease tensions.

The Allied argument emphasised that Austria-Hungary and Germany were the only two states looking for war in the summer of 1914. The Habsburg leadership, as the British foreign secretary Edward Grey noted on 27 July, set itself intransigently on course for a conflict with Serbia - even though it ran the risk of drawing in Russia. The unequivocal support offered to Austria on 5 July 1914 by Kaiser Wilhelm II and the German Chancellor Theobald von Bethmann-Hollweg gave the Habsburg monarchy the 'blank cheque' that it needed to proceed with military action against Serbia.

Germany - far more than Britain, France or Russia - was prepared to risk a general European war in order to safeguard and expand its position as a Great Power. Russia's general mobilisation provided the pretext for the German military command to order the mobilisation of its own armed forces, a move that - given the aggressive nature of the Schlieffen Plan - inevitably meant war.

Though the July crisis provided the immediate spark for war in August 1914, its coming was also influenced by long-term factors. From 1907 onwards, Europe was divided, clearly if informally, into two military blocs: the Triple Entente (Britain, France and Russia) and the Triple Alliance (Austria-Hungary, Germany and Italy).

Imperial and military rivalries exacerbated the mutual mistrust that characterised this secretive system. Both sides coveted the territories of the Ottoman empire. The scramble for new colonies in Africa and elsewhere contributed to the escalating Anglo-German naval race. This in turn encouraged a general rise in military expenditure. The French, German and Russian armies all significantly increased in size during the early part of the early 20th century.

On the surface at least, the omens for peace in Europe seemed promising in 1914. Anglo-German relations in particular seemed to be improving: in late June, the British naval fleet even visited its German counterpart at the port of Kiel. On all sides, domestic issues were to the fore: the Irish problem in Britain; the trial of the second wife of Joseph Caillaux in France; and threatened labour unrest in Russia.

The assassination of Archduke Franz Ferdinand on 28 June, however, quickly revealed the fragility of this apparently serene state of affairs. Within the space of five weeks, most of Europe was at war. The military planners in both Britain and Germany - who had long predicted, and in some cases championed, the prospect of such a conflict - were finally confirmed in their fatalistic assumptions.