I have just walked from mid-town down to the area, drawn like a magnet to where we once called home. The morning was a beautiful bright blue autumnal New York day, cool and crisp and just right for a brisk outing. I went through Central Park with its late-Fall colours and then meandered my way along Broadway and all the way down West Street Highway going south, past the weekend market where we used to buy fresh farm produce brought in from upstate New York.

By the time I reach Ground Zero it is near nightfall and suddenly the day has turned to a dark and sombre mood, almost surreal as the lights light up the scene. You can get to within two blocks of the spot, and see quite clearly what is going on. The demolition work continues, around the clock. The hoses are still playing, and the crane hovers over the site, looking unwittingly like a vulture in for the pickings. The demolition ball rises and falls, with a dull thud onto the concrete slabs hanging at grotesque angles on the stumps of the two buildings. There are eight stories left, open and gaping to the outside world, tattered and ugly and caught in the lights like a toothless hag. A small crowd of Americans stands at the perimeter, silent and pensive, with muted voices and grim looks. There are bodies still inside, many of them.

I turn towards the river and walk down our esplanade along the Hudson where we ate outdoors each night watching the sunset. The neighbourhood damage is plain to see. The Winter Garden, that superb indoor space with palm trees and glass roof and front looking onto the river, is now a shell, with the glass blown in and the beams exposed. So is World Financial Centre Number 4, right across the road from our apartment. The southeast corner of the Financial Centre Number 2 is ripped open. Then, along the esplanade, by the water concourse, is the memorial that has sprung up - private testimonies to the loved ones, who lie there, still, close by.

It is a poignant scene, sad and sombre where there used to be skating, blading, jogging and laughter. Now in its place are flowers and photographs, children's poems, letters to fathers, and above all the teddy bears - hundreds of teddy bears. A few life stories. One father, it says, had worked in Washington all his career. He had just retired and had gone to work as a security agent for a firm in one of the Twin Towers. He had commenced work on 10 September.

I walk around onto South End Avenue, our avenue, and into our apartment complex, with its once beautiful entrance. I go inside and meet the doorman, a huge and muscled fellow from Panama. Yes, he had been on duty on 11 September. He saw a wall of blue-grey smoke coming fast at him. He turned around, shot out the back exit onto the esplanade and leapt into the Hudson. It is a big and fast-flowing river down by that spot. But he was a good swimmer and got a third of the way across before a Coast Guard cutter picked him up.

I go back onto South End and down Rector Place where John McEwen used to live. At the end there is another small group, looking back on the devastation towards the north, silent. That is when you get a glimpse of the modern icon of wanton mass destruction - the eerie silhouette, the structure that the global media has made famous, lurched over on an awkward angle, pointing high, still, in supplication to the heavens. It shows up silver against the dark blue sky as night takes over, and the searchlights glare into the ruins, inappropriately, for the land of the dead.

I retrace my steps back around the river and onto the north side once more, taking one last look south. Again, the darkened hulk of two once sleek and gleaming structures, the hose playing, the ball thumping methodically on unfeeling concrete, the silent crowd reflecting upon themselves, their surroundings and their fate - and, above it all, a clear crescent moon looking softly down.

If ever there is a powerful case for strong and enlightened leadership to steer humanity through the shoals of our own misfortunes and mistakes, not just in North America but around the planet, we are experiencing it now. History will judge us by the quality of our leaders - the legacy left behind of societal harmony and environmental integrity - this generation and the next.

The instinct for revenge and justice having been satiated for most in Afghanistan, let us reflect on what September 11 means for our modern age. Hiroshima was defended by the US president in the name of revenge and military necessity. "Having found the bomb", explained Truman, "we have to use it. We have used it against those who attacked us without warning at Pearl Harbour, against those who have starved and beaten and executed American prisoners of war, against those who have abandoned all pretence of obeying international laws of warfare. We have used it to shorten the agony of war, in order to save thousands and thousands of young Americans." Some 150,000 civilians were immolated without warning in Hiroshima and Nagasaki, the final act in a genocidal war that knew no bounds of military restraint, no limitation on group hatred. The trials of Nuremburg and Tokyo brought some measure of relief, yet half a century later we still await a permanent court to ensure an objective justice for our future misdeeds.

The possession of weapons of mass destruction still poses the greatest immediate threat to humankind, even in an age of climate change and global resource stress. The key, in the longer term, lies not in the major powers insisting on their sole retention, on grounds of superior political wisdom and safer command and control - still less on a moral justification by faith. The genie will not be so contained. The Non-Proliferation Treaty, that bastion of Cold War stability, did not corner all nations. Still less did it place the dragnet over private groups.

They will come to possess weapons of mass destruction in the future - the disaffected and the aggrieved of humanity - and they will quite likely use them in an act of blind outrage, as outrage it will be. Six billion going on ten, with inequality of wealth at 70 to 1, ethnic and historical discord, resentment over whatever global imperium prevails, and the chance to sow the seeds of change, no matter what the outcome.

We shall not be safe from ourselves unless and until we lance the source of such grievance, and unless we move against the national retention of such weaponry. The NPT spoke not of the elimination of nuclear weapons from human hands, but from the national arsenals of states. In the late 1940s, those days of relative innocence, the international community toyed with the idea of international possession of atomic fission. But the idea collapsed, through mutual mistrust among the major powers at the time.

Half a century on, we are about to complete the nuclear parabola with far-reaching force reductions towards the level of minimal deterrence. Then the moment of truth will be faced - a decade from now or a century. Can we place such weapons in the hands of the global sovereign, however configured and legitimised - can we trust ourselves that far?

Even then, the genie will remain loose among the entire human race. The threat of a disaster of our own making will be ever-present so long as inequality and resentment, inflamed by fanatical belief, exist. But it would at least be contained in time and place. And we shall have moved to the state of a global society where law exists and seeks to prevail, where an enlightened level of compassion for all equates with a sobered recognition that there can be no other way. For the burden of nuclear knowledge, the pain of inequality and injustice, and the instinct for human aggression and revenge will forevermore exist side by side. For humanity to climb out of the abyss, we must confess our collective guilt, and strive anew for redemption through an appreciation, not of the ideal state that is beyond our reach, but of the imperfect global society that is within it.

Kennedy Graham, a NZ national, is Director of the United Nations University Leadership Academy. These comments, contributed in a personal capacity, do not necessarily reflect the views of the United Nations.