The multiple nuclear tests by India, and Pakistan's responding detonations, are among those rare events that redraw the map of the world. A terrifying nuclear arms race in south Asia is likely, with the distinct danger of "nuclear battle" between India and Pakistan.

North Korea is reneging on agreements to yield key nuclear materials. China is contemplating a full nuclear security guarantee to Pakistan, creating a complex, unstable four-sided nuclear stand-off in Asia between India, Pakistan, China and, of course, Russia.

Nuclear war, many regional experts say, is more likely now than it ever was in the Cold War. Unlike Russians and Americans of that bygone era, south Asia's peoples actually have been killing each other over the years, by the millions during the partition of India in 1948, and in three wars since then. Moreover, India and Pakistan remain divided by a bitter territorial dispute over Kashmir. But this possibility of nuclear war, horrifying as it is, may not be the most important consequence of India's test. That consequence is the gaping hole punched in the (little-known) system the world has relied upon against nuclear dangers in the post-Cold War era. Like sleepers awakened by a shock, people across the globe are startled to discover that they are still living in the nuclear age.

That little-noted system of protection is the one mandated by the Nuclear Non-Proliferation Treaty of 1968, signed by 185 nations, which created two tiers of states. The first consisted of the five nations possessing nuclear weapons (the US, Russia, Britain, France and China), which promised (under the treaty's Article Six) to get rid of them. The second consisted of nations lacking nuclear weapons, which promised not to get them.

The NPT was an abolition treaty, specifying that non-nuclear nations would start at the goal of zero nuclear weapons, and that nuclear-armed nations would commit themselves to reaching that goal. At some point the two tiers would fuse, and the world would be nuclear weapons-free.

Nothing like this occurred during the Cold War. Nuclear-armed nations built nearly 100,000 weapons, bearing the explosive power of about 1,600,000 Hiroshima-size bombs. With the Cold War's end, arms reductions began, but the nuclear powers - the US in particular - publicly disavowed total nuclear disarmament for "the indefinite future", in the recent words of Mr Robert Bell, of the US National Security Council. The US and the other nuclear powers proposed to keep the two-tier system in virtual perpetuity.

This is the context in which India's catalytic folly and peril took place. ("The news of the day is: India defies world opinion," Prime Minister Atal Bihari Vajpayee proclaimed with insane arrogance.) India was in effect saying that if there were to be two classes of nations in the post-Cold-War world, then India was going to be in the first class. It would be "a global player", in the words of Mr Pramod Mahajan, an aide to the Prime Minister.

Pakistan, facing huge domestic pressures to demonstrate its nuclear prowess, defied its own economic self-interest to "settle the score", in the words of Pakistan's Prime Minister, by a show of atomic force.

BOTH daring gambits make perfect sense if you assume that the post-Cold-War world is going to remain nuclear-armed and that nuclear arms will remain the main currency of power. But is that to be the case?

Do the nuclear powers intend to hold on to their nuclear arsenals "indefinitely"? Or are these powers (now including India) prepared, by joint agreement, to commit themselves to creating a world without nuclear weapons? This is the question India has placed before the world.

India's and Pakistan's tests mean that the days of the two-tier system are over. The world is moving toward a single standard, and the question is whether that standard will be possession of nuclear weapons or renunciation of them.

While India first posed this question, the two nations of the subcontinent cannot answer it. The world, pre-eminently the US, must do that. If President Clinton were to embrace the goal of a world without nuclear weapons (or other weapons of mass destruction), there is reason to hope that other nuclear powers would follow. Welcome as American sanctions against India are, the most important response to the Indian tests would be an international debate.

Indeed, a considerable number of the most prominent figures of the Cold War, in Russia and the US, support abolition of nuclear weapons. They include the former German chancellor Helmut Schmidt; Paul Nitze, considered by many the author of America's Cold War nuclear policy; former NATO commander Andrew J. Goodpaster; and Russia's Gen Aleksandr Lebed.

While proponents admit that a world without nuclear weapons would not be without dangers - especially from governments that might decide to violate abolition agreements - these dangers rate far below the perils of living in the world of uncontrolled nuclear proliferation toward which the Indian tests point.

In announcing his sanctions to India's leaders, President Clinton said: "They have to define the greatness of India in 21st-century terms, not in terms that everyone else has rejected." But have the US and the other nuclear powers really rejected those terms? Are they ready to follow the advice given to India and Pakistan and let go of their nuclear arsenals? The great nuclear powers should demonstrate their resolve not only in words, but in deeds.

Jonathan Schell was for 20 years the political editor of the New Yorker, and is the author of the global best-seller The Fate of the Earth. His latest book is The Gift of Time: the Case for Abolishing Nuclear Weapons Now.