The big changes came with the First World War, which put millions of women to work in factories and led to widespread questioning of the nationalism from which the conflict sprang. Yet although women began to get the vote in the early Twenties, they did not enjoy genuine equality with men until after the Second World War, as Virginia Woolf demonstrated in her seminal tract, A Room of One's Own. And despite the creation in 1920 of the League of Nations, with its concept of collective security, nationalism remained the decisive force in world affairs.
Though Communism, represented by the Soviet Union, dominated a fifth of the world, it failed to overcome nationalism, as Mao Tse Tung in China and Marshal Tito in Yugoslavia were to demonstrate.
The competing ideologies that produced Communism in Russia and Fascism in Germany and Italy needed force to maintain their totalitarian regimes. But it was the failure of free-market capitalism to produce social justice and prosperity that made the victory of totalitarianism possible in those countries. However in Germany after the First World War the refusal of the victorious allies to help its economy also played an important role.
All this, and the refusal of right-wing European governments to honour their commitment to collective security, made the Second World War inevitable. That war ended with Europe divided through the middle of Berlin and Vienna, and the Soviet Union imposing Communist regimes throughout Eastern Europe, in defiance of its wartime promises to permit free elections there. The Cold War became unavoidable.
Looking back, we can now see that the first great milestone of the century was the entry of the United States into the First World War, almost at the same time as the second great milestone: the storming of the Winter Palace by the Bolsheviks in St Petersburg.
The entry of the United States into the Second World War was not inevitable. Indeed the US Congress had passed a Neutrality Act in 1936 which in theory forbade it. Even after the Japanese attack on Pearl Harbor, President Franklin Roosevelt might have confined the US's military activities to the Pacific theatre. And Stalin had signed a Non-Aggression Pact with Hitler; it was only Germany's attack on the Soviet Union which turned the latter into an ally of the West.
But it was their victory in the Second World War that made the United States and the Soviet Union global superpowers, a position immeasurably reinforced by their possession of nuclear arsenals, which guaranteed that the Cold War could never become a Third World War (though even today the possibility of war between smaller nuclear powers such as India and Pakistan cannot be discounted).
It did not take long for Russia to realise that there was no chance of Communists taking over in any Western country, hence the Soviet Union's adoption of the policy of "Peaceful Co-existence". It still believed after the war, however, that Communism might yet triumph in the Third World, the developing countries of Africa and Asia that were emerging from imperial rule by the Europeans. Yet it soon became obvious that the former colonies had no intention of replacing European rule with rule by the Russians. The few that flirted briefly with Communism did so in the vain expectation of massive Soviet aid.
The collapse of the European empires after the Second World War reflected a desire for national independence. In Africa this led to new wars, since colonial frontiers simply represented the lines at which the different imperial armies had happened to meet, so usually cut across ethnic or tribal frontiers. Even in Europe some national frontiers simply ignored national or religious feelings, as in the Saarland, and much of Eastern Europe, where the Austro-Hungarian empire had believed in "divide and rule".
The post-imperial world was shaped significantly by religious conviction. Yet the major world religions contained their own deep divides in the form of sectarian quarrels. Conflicts between Catholic and Orthodox Christians still trouble politics in the Balkans; those between Sunni, Shia and Wahabi Muslims have made Arab unity a fantasy in the Middle East.
From the Eighties, technology - and information technology in particular - has compelled us all to look beyond the nation-state. Even the most securely established absolute monarchies, such as that of Saudi Arabia, can bIe threatened by e-mails from London. And as the idea of democracy takes root among developing nations, it is becoming harder for regimes to survive if they permit vast differences in rights or incomes. Moreover, economic changes are beginning to make people look for political change.
The world's population has increased four-fold in the last hundred years, as have real incomes per head. In India real incomes per head have increased more than 150 per cent since independence. In Russia, on the other hand, real incomes have fallen for most people since the end of Communism, while crime and corruption have risen to unprecedented levels. So the United States is now the only global superpower.
One of the most intractable problems facing Western governments is to decide whether they should do anything about the troubles of countries such as Russia, and if so, what. The consequences of doing nothing could be dangerous. Armed conflicts in the Caucasus or south-central Asia might well spread to areas of direct concern to the West. Yet the problems faced by the IMF in getting Russia to fulfil the commitments it made in return for aid illustrate the difficulties facing potential donors. Kosovo has shown us the difficulties of military intervention, particularly by countries reluctant to use ground forces for fear of seeing body-bags on their TVs.
The last 10 years have brought one great triumph for humanity: the end of the Cold War and the reunification of Germany and Europe. This was not, as so often claimed, a victory for the West. It was down to the decision of one man, Mikhail Gorvachev, to withdraw the Red Army from Eastern Germany - something that all Western intelligence agencies said could never happen. Now the West faces its real challenge: to accept the former Communist countries of Eastern Europe into the European Community. Their membership of Nato should be considered more cautiously in case it leads to damaging consequences inside Russia or the Ukraine.
These problems are better seen within the context of the growing globalisation of world finance, economics and politics. Information technology has made it possible to move billions of dollars from New York to Tokyo in microseconds. Yet the dependence of info-tech on computers makes this process vulnerable to hacking and blackmail through the planting of computer viruses. Moreover the creation of new financial instruments such as derivatives and hedge- funds has reduced the reliability of economic theories. Hardly a day passes without the falsification of some economic formula that had hitherto been taken for granted.
In any case economic forecasts, and the theories underlying them, have throughout the age been upset by unforeseen political events. None of us predicted Suez, the Vietnam War, the revolution in Hungary, the emergence of Charles de Gaulle as president of France, or the creation of Opec. Few of us foresaw either the rise of Japan as the fastest growing country in the industrial world, or its recent decline into "the sick man of Asia". So it is wise to approach economic forecasts with a healthy scepticism. A good example is the OECD's recent forecast of an economic boom that could last for a quarter of a century, which is said to depend on "saying goodbye to the relatively static, secure and lifelong form of work ... requiring a specific and known set of skills".
Increasingly the advance of science sets much of the agenda, both at home and abroad. More than 90 per cent of all the scientists who ever existed are alive and working today. They already offer us the choice between the elimination of poverty and the elimination of the human race. Yet we are extraordinarily capricious in deciding how to use the knowledge they give us.
Thus we worry about eliminating BSE, which infects one in a million of those who eat beef on the bone, yet do nothing about smoking tobacco, which infects about one in ten of its addicts with cancer.
One thing at least seems fairly certain. Most of the problems we face today are not susceptible to purely national solutions. It may be possible to solve some of them through an untidy proliferation of overlapping organisations whose memberships are determined by the nature of the problem. But it is difficult to believe that this will work without some overriding organisations which include all countries, such as the IMF, the World Bank, and of course the United Nations, the successor to the League of Nations that was established by the victors after the Second World War.
It is clearly unwise - as well as wasteful - to have more than one such organisation covering much the same field; hence the IMF and World Bank's discussions about some form of merger. But the increasing globalisation of all activities that affect more than one nation-state points to a restructuring of the United Nations itself, to better reflect the enormous changes we have gone through since 1945. Membership of the UN Security Council no longer represents the realities of power today, and it is difficult to justify a General Assembly in which a tiny island in the Caribbean has the same weight as Germany or Japan.
If the world is indeed facing 25 years of sustained growth, there could be no better time to undertake the gruelling task of bringing our international organisations into closer touch with the realities and challenges of the 21st century.