Arguments over Iraq created a damaging rift among Nato allies
Disagreements over Iraq led to a bitter confrontation between old allies. The US found it could no longer rely on the unquestioning support of countries who had been its partners through the Cold War and beyond.
When you are in the thick of things, you rarely see the curve in the road ahead which is about to mark a momentous shift.
We journalists, watching some drama unfurl close up and trying to make sense of all its confusing detail, shrouded in foggy claim and counterclaim, are often not best placed to judge if this is indeed a defining moment.
But sometimes there are moments that imprint themselves on your memory .
A doom-laden thought flashes by: "This is it. The world is about to change."
Most recently for me it happened, of all places, in a stuffy grey basement room at the United Nations, when the political wrangling over the Iraq war was coming to a head.
The UN building always hums with a multi-coloured kaleidoscope of diplomats, flitting back and forth through its labyrinthine corridors, riding up and down the escalators with a pre-occupied air.
I always used to suspect that many only pretended to be so busy, that really they were on their way for a smoke, or a newspaper, or a quick snack and a gossip in the bountiful canteen.
But in the early part of this year the atmosphere was different.
Suddenly the United Nations mattered.
The UN Security Council was the world's most important theatre, where leading actors jostled for the limelight, each seeking to win over the global audience.
UN SECURITY COUNCIL PERMANENT MEMBERS
The occasion was a session summoned to hear Dr Blix's latest report on his weapons inspectors.
So intense was the disagreement over Iraq, that all the Security Council foreign ministers had flown in too.
No-one wanted to risk an opponent grandstanding on his own.
I was in the basement because it was the only way to watch proceedings.
The Iraqi crisis tested the Security Council's authority to the limit
Sitting on an upturned metal box, in front of a television monitor that was relaying speeches live from the Security Council floor, I was writing down the important parts in my notebook.
It was a dusty room full of cardboard cartons, old furniture and the usual untidy journalistic mess: empty plastic teacups and water bottles, some cold half eaten pizza from the night before, coils of wire and boxes of tape, and never enough chairs.
The session was full of drama.
Dr Blix failed to chastise the Iraqis as much as the Americans wanted and claimed some of the US intelligence on Iraq was downright wrong.
Worse, the chief nuclear inspector, Dr ElBaradei, declared he had found no evidence of an Iraqi nuclear programme at all.
It felt like a muted gesture of open revolt
Glowering, Colin Powell, America's secretary of state, launched into an off-the-cuff performance, almost pleading with the council and the millions watching round the world to accept the case for war.
I scrabbled to write fast enough, puzzling over where this would lead.
The French Foreign Minister Dominique de Villepin, another formidable orator, took the floor.
His speech was equally ardent, arguing that the world did not necessarily have to follow America's lead.
Then something extraordinary happened.
As he finished there was a ripple of applause. Not something usually allowed in the Security Council chamber.
It felt like a muted gesture of open revolt.
Down in the basement, I thought, I shall remember this moment.
It wasn't the first crack in the transatlantic alliance.
Dominique de Villepin's speech drew rare applause at the Security Council
There had already been angry rows at Nato meetings and in the European Union.
But for the first time it seemed to me that it might not be possible to patch them up.
The applause had signalled that France was not isolated.
Opposition to the war was echoed not just by Germany, but China, Russia and others.
One Russian diplomat I met a week later was positively gleeful.
"This is how it should be," he said, " Europe allied not to the United States but to Russia.
"One day maybe Russia will be the leading voice in Europe. After all, geography is on our side."
Of course in the months since the Iraq war ended, diplomats, especially British diplomats, have tried hard to argue that no real damage has been done.
Bitter disagreements, they say, have given way to a new spirit of unity.
"Look at the recent resolutions on Iraq passed unanimously at the United Nations," they say.
"Look at Nato's new role in Afghanistan. And look at the deal to limit Iran's nuclear programme which Britain, France and Germany brokered between them. European conciliation is winning out over American confrontation."
But the resentment has gone deep.
The wounds have not really healed.
Why else would the US say countries allowed to bid for American-funded contracts to rebuild Iraq will embrace peripheral allies like Micronesia, Eritrea and Uzbekistan, but not key Nato partners like Canada, Germany and France?
When George Bush visits Europe, anti-American protests erupt with alarming regularity.
So disturbed is Washington that it is rethinking where to position its military bases.
And in the US it's the same in reverse.
"If you see any Frenchmen, give them two fingers from me," growled one American customs officer, making the offensive gesture, as he checked my passport the last time I went to New York.
It feels as though the tectonic plates are shifting.
Even when the Cold War fizzled out with the collapse of the Soviet Union in 1991, the old established order that had been there since the 1950s still held firm.
The US and its allies in Western Europe were still the bedrock of our world.
It's no longer that simple.
From now on, global alliances will be much less easy to second guess.
From Our Own Correspondent was broadcast on Saturday, 27 December, 2003, at 1130 GMT on BBC Radio 4. Please check the programme schedules for World Service transmission times.