For Britain, it broke out of a near cloudless sky. In May 1914, Colonel Edward House, President Wilson's emissary, had visited Berlin. He found the 'militaristic oligarchy' supreme, 'determined on war' and prepared even to 'dethrone the Kaiser the moment he showed indications of taking a course that would lead to peace'. Reporting to London, House tried to convey this feeling to Sir Edward Grey and other members of the British Government. 'They seemed astonished at my pessimistic view and thought that conditions were better than they had been for a long time,' he said.
After the murder of the Austrian Archduke, Franz Ferdinand, on 28 June, the Commons debated foreign affairs in an ill-attended House at which the main question of interest was the Anglo-Russian dispute over Persia. A pacifist Liberal backbencher congratulated the Foreign Secretary that 'matters which are likely to be raised are not questions of haute politique at all, but they are comparatively minor questions not involving matters of great danger'. On 17 July, two-and-a-half weeks before Britain declared war, Lloyd George told a City banquet that relations between Britain and Germany were better than they had been for a long time, and urged disarmament.
The Liberals held that conflict in the Balkans could be resolved through negotiation, while Labour believed that international working-class solidarity would prevent war. Both were wrong, because they underestimated what has proved to be the most powerful force in 20th-century Europe: popular nationalism.
Poland's inter-war leader, Jozef Pilsudski, once declared that he had got on to a train whose destination was socialism, but had left it at the station marked nationalism. Pilsudski's journey symbolises the 20th century. In the 19th century, Karl Marx had predicted class war and revolution. Yet in our own century classes have collaborated while nations fought.
With war inevitable, the Kaiser, in August 1914, received the German Social Democrats, who had just voted for war credits, for the first time. 'I no longer recognise parties,' he declared, 'I recognise only Germans.' In Britain, Labour repudiated its pacifist leaders, Keir Hardie and Ramsay MacDonald, and endorsed the war as enthusiastically as its Liberal and Conservative opponents. Everywhere the masses, whom Marx had seen as revolutionary, found that they had more in common with the ruling classes of their own country than the workers of another.
It was during the First World War that a young Italian journalist, Benito Mussolini, abandoned socialism to become an Italian nationalist, while, at the end of the war, Hitler found his vocation as a National Socialist. They ensured that the First World War, far from being 'the war that will end war', in HG Wells' famous phrase, was the prelude to more extreme forms of popular nationalism that came to be dignified with the titles of Fascism and National Socialism.
During the inter-war years, popular nationalism destroyed democracy in the new nation-states of Central and Eastern Europe. By the mid-Thirties, all of them except Czechoslovakia had abandoned parliamentary democracy. Indeed, when in 1931 Spain reverted, albeit briefly, to parliamentary government, Mussolini declared that this was 'going back to oil lamps in the age of electricity'.
In Europe east of the Rhine, the people were embracing dictatorship, most notably in Germany, where Hitler came to power in a democratic and constitutional order, voted in by the people and sustained by their enthusiasm.
The First World War yielded a second form of dictatorship, ideologically distinct from Fascism and National Socialism, yet bearing more than a few resemblances to these movements - Communism. For Communism, too, in its Stalinist form, was a nationalist movement, designed to show that socialism could be created in one country and that Russia could overcome her traditional backwardness. The Bolsheviks represented an indigenous form of socialism against their cosmopolitan opponents. The young Trotsky once called them slavophilising Marxists.
In the 19th century, nationalism had been seen as a liberal force. Giuseppe Mazzini, the Genoese propagandist and revolutionary, had looked forward to a Europe of independent nation-states freely co-operating together for the good of their peoples. But the 20th century has proved inhospitable to such hopes. Nationalism came to be transformed into a radical force. Indeed, the combination of nationalism and democracy proved so explosive that it almost destroyed European civilisation.
We have yet to outgrow the legacy of 1914. For the sentiments that Woodrow Wilson legitimised after the First World War have, in the new democracies of Central and Eastern Europe, far outlived those inspired by Marx and Lenin. Popular nationalism, long suppressed by Communism, is now a dominant force. Not only has it destroyed the empire created by the Tsars and restored by Lenin and Stalin, but it has also broken up the two multinational states created by the peace treaties of 1919 - Czechoslovakia and Yugoslavia. In Croatia and Slovakia, there is growing nostalgia for wartime nationalist leaders, while in Hungary, Miklos Horthy has been given an honourable reburial and in Romania, streets have been renamed in honour of the wartime dictator, Marshal Antonescu.
The western half of the continent is not immune to popular nationalism either. There are signs of a revival of the radical right in Austria, Belgium, France and Germany, while in Italy a party that openly admits to admiration for Mussolini has just taken its place in government, to the plaudits of the British Foreign Secretary.
Many people have spoken of the changes in Europe since 1989 as having created a new political order. It would be more accurate to view what has happened as the restoration of an old order, which the peacemakers at the end of the First World War tried to create to fill the vacuum left by the decay of empire.
The slogan of the peacemakers in 1919 was national self-determination. The problem with the notion is that in Central and Eastern Europe - and in Ireland, too - where majorities are intertwined with minorities, there is no way of creating homogeneous national communities by drawing lines on a map. Wherever the line is drawn, significant minorities will be left on the wrong side. For this reason, the principle of national self-determination is not part of the answer to the question of how states should be organised, but rather a large part of the problem.
In 1914, it was the growth of nationalism among the Balkan Serbs which lit the fuse that set Europe alight. So also today, the forces of ethnicity could easily undermine the nascent democracies in Central and Eastern Europe.
Woodrow Wilson's notion of self-determination was founded on the 19th-century liberal idea that humanity was naturally divided into nations and every nation should have its own state. Such an ideal is clearly not capable of realisation in Central and Eastern Europe, where minorities are territorially dispersed. This means there is an urgent need for new thinking as to how the national identities of peoples can be made compatible with democratic stability.
Western Europe has sought to transcend nationalism through European union, the impulse to which was kindled in the ashes of the Resistance. The fundamental leitmotiv of the European Union is power-sharing between states. So also, in Central and Eastern Europe, the answer to the problems caused by the intermingling of different national groups lies not in national self-assertion, but in power-sharing. That involves downgrading the concept of sovereignty, whether it be the sovereignty of the majority within the state, or that of the state itself vis-a- vis other states. And it involves applying the concept of power-sharing within the state, so that the identity of minorities can be safeguarded, and between states, so that national conflicts can be contained within a wider multinational framework.
The history of Central and Eastern Europe since 1914 can be seen as an extended commentary on two themes: the idea of national self-determination legitimised by Woodrow Wilson, and the Bolshevism of Lenin and Stalin. The new democracies have overcome the legacy of Lenin and Stalin. Will they also overcome the more insidious legacy of Woodrow Wilson?
The First World War was at root a war of national self-assertion. Only when Europe is able to transcend national self-assertion will it exorcise the legacy of 1914. Only then will it have overcome the 20th century.
The writer is Reader in Government, Oxford University, and a Fellow of Brasenose College. This article is based on the Mishcon lecture 'Overcoming the 20th Century' delivered at University College London in May, which is to be published in the 'Political Quarterly'.