It wasn't just London -- Damascus, Athens, Seoul, Rome, Tokyo, Sydney - hundreds of cities worldwide witnessed the same thing.
I imagine for many people alive today, the great politicising event of their childhood came in the form of a tragedy.
The first dreadful hammer of the Luftwaffe passing overhead, the panicked screams at the Dealey Plaza or the bullet holes at Bogside - that key event that propelled you to develop a political consciousness seems more likely to have been one that made you angry than inspired.
But for many of us who were still just 16 on 15 February, 2003, that landmark came in a moment of hope when more than 1,000,000 people descended on the streets of London to march in protest against the imminent invasion of Iraq.
It was, we were told, the largest public protest in British history. I still remember the feeling of pride as I poured over the pictures, that sense that we belonged, not to the most 'politically apathetic generation' ever to live after all, but to the most engaged, the most righteous.
Like hundreds of teenagers who didn't make the real thing, students at my school hastily arranged their own small protest, marching through our small rural town chanting and playing anti-war music.
We must have looked pathetic, but we didn't care. We were adding a cry to a national roar that made the hairs on the back of our necks stand up.
It didn't even occur to me that it might not work.
But first, let's go back.
Perhaps growing up in a small town in Northumberland - the UK's northernmost, most sparsely populated county - we were a touch parochial. But the events of 9/11 didn't so much politicise my schoolmates as demonstrate to us for the first time that there was a wider world out there at all. Most of us didn't know what the Twin Towers were until they were crumbling in a plume of smoke. None of that 'stuff' - the news, the bickering politicians - had any relevance to us.
And then in the space of a day, before the dust in Manhattan had even settled, the notion of politics had sprung up out of nowhere, like that moment when you realise the opposite sex isn't just attractive but that they're going to matter, a lot, for the rest of your life.
I began reading newspapers to find out what was going on. I bought the Times because I had heard of it, until an older boy said it was 'right-wing'. I figured out what right-wing meant, and bought the Guardian.
The space between 9/11 and the Iraq War being something people were talking about felt tiny, and that's because it was. Just nine days after the attack, George W Bush, addressing Congress (and the world), uttered the words 'War on Terror' for the first time. Fabrications about the links between Saddam Hussein and al-Qaeda followed, and the international reputation of the most despised and ridiculed American in living memory was launched.
We couldn't have known, at the time, how 'Dubya' would come to dominate how we felt about the world for the next eight years, how he'd oscillate from a figure of fun for our blossoming liberal sententiousness to quite simply the most terrifying man in the world. How he'd eventually make us despise our own prime minister. How, because of him, shamefully tossing around prejudice remarks about 'stupid Americans' felt OK.
The crazier Bush's rhetoric got, the more he divided the world into good and bad like it was some kind of He-Man cartoon, the more I clung to the notion that here, we wouldn't be drawn into using phrases like 'axis of evil'. America was excitable, in the throes of imperialism. We were post-empire, cynical and weary.
But our prime minister saw things differently. Up until 2001, I think most of my generation still believed, in an abstract way, that Tony Blair was a decent man. He looked and sounded good on tele, he'd ended the conflicts in Ireland that had been a distant bogeyman of our childhoods and the other guy - the Tory -always seemed old, bald and well, boring.
But now suddenly, Blair was siding with Bush at every turn. When the president launched his War On Terror, Blair said he'd back it. When the president said he believed Iraq had weapons of mass destruction, Blair said he believed it too. The press presented him as Bush's poodle, and we winced in acknowledgement.
Then came Resolution 1441 and Hans Blix. Blix swept into the darkening saga like a comforting beam from a lighthouse. The arrival of the peaceful Swede, with his glasses and nervous smile, seemed to my young mind like democracy at work. All Iraq had to do was open to doors to the weapon inspectors, show they had nothing to hide and war would be avoided. Like Piggy from Lord Of The Flies, Blix was supposed to be the rational voice of intelligence. But like Piggy he was taken out of action by an unstoppable boulder: an American government that had made its mind up to go to war long ago.
Blix didn't find a thing, because there were no WMDs to find. By 31 December 2002, his team had reached the same conclusion as an Iraqi dossier presented to the UN during the same period: they were in the clear. It should have ended right there. Instead, two years later, Blix would tell the BBC what by then we all already knew - Bush and Blair ignored him and dramatised a threat in order to start a war.
Looking back today, the whole charade that precipitated the Iraqi War reminds me of an incident from my teenager years.
A friend of mine had developed a complex about never having been in a fight. None of us had, really, being comfortable, middle-class kids who had grown up with no need for violence. One night this friend got drunk and followed an even punier guy from school out of the pub, where he backed him into a corner and preceded to bait him.
Trying to pluck up the courage to take a swing, my friend taunted this other kid unsuccessfully, before trying a new tact. He started sexing up some perceived slight from long ago, wanting to convince himself the guy deserved what was coming to him. He worked himself into a faux outrage, then finally, after an excruciating half hour, threw his punch.
My friend, you might say, was like a new American President desperate to prove he could be tough by attacking an opponent he knew he could easily beat. And in the background, feeling uncomfortable but doing nothing to stop it, was me. Tony Blair.
Bush and Blair pushed for a second UN resolution to start a war in Iraq and failed, but American and British troops continued to build up around the Gulf. On 7 February, Downing Street admitted that its dossier on Iraq - released the previous week to push the case for war - was a muddled patchwork of academic sources pulled together by mid-level lackeys of Alastair Campbell. Still, the troops flooded in. Three days later, France and Germany make a last ditch attempt to keep the peace by suggesting the UN triples the number of arms inspectors in Iraq. The US-UK alliance ignored them.
Which brings us to the day of 15 February, 2003 - the day of my generation's political awakening.
For two years we had watched our government join America in ignoring every plausible reason for not starting a war with innocent people in a poor country in the Middle East - the scepticism of the press, the will of the UN, the weapons inspectors, the facts. It had been a depressing lesson in the limitations of politics and politicians, these things we'd only just started paying attention to.
Then, at what felt like the last moment, the people stepped up.
Early in the morning they assembled: first at Embankment, then, through force, Westminster and Whitehall. Ken Livingstone, the mayor, led the way. Over a million people - a million, all sending a message to Tony Blair: you work for us, and we don't want this war. And it wasn't just London, either. Damascus, Athens, Seoul, Rome, Tokyo, Sydney - hundreds of cities all over the world were witnessing the same thing.
280 miles away from London, we were feeling for the first time the age old thrill of protest. A handful of us skipped school in the afternoon, drew CND signs on our face with one of the girl's eyeliner and took a portable stereo playing Bob Dylan and Barry McGuire to our town square. Shoppers wandered by looking bemused. Some stopped to tell us well done. We argued with anyone who would listen about the lack of WMDs. We stood beneath a gloomy overcast sky but felt bathed in a warm glow.
Guardian/ICM polls at the time put support for the war at just 29% of the public, with 52% opposing. But Blair heard about polls all day long. Naively, I thought a million people marching past his window would be impossible to ignore.
A little over a month later, at 9.34pm on Wednesday 19 March, we watched on television as the first bomb fell on Baghdad. 28 British soldiers would die before the month was out.
If every generation lives through an event that opens their eyes to politics for the first time, then perhaps there also an event that closes them again, if not entirely then at least in part.
My father isn't a political man. He's passionate and he cares about lots of things, but the political system depresses him. All my life I've heard him dismiss it - 'they're all the same', 'it's pointless' - and describe himself, with a hint of sadness, as 'apolitical'.
In the build up the Iraq War, and particularly on the day the world marched, I couldn't have understood his stance any less. I remember feeling disappointed in him. But as it came to dawn on me that it had all come to nothing, that Blair was pushing ahead with the war anyway, I came to understand a little of how he felt.
It left me - and most of us at school who had taken an interest in the world after 9/11 - bitterly angry. The buds of our idealism were buried under an avalanche of cynicism. I couldn't comprehend how Blair had the nerve to ignore the will of the country. I assumed that, if they'd ignore us over something as important as war, they'd ignore over anything.
Ten years later and, like my father, I care about the issues politics affect, but I don't trust the political system, and I don't believe in politicians.
And I'm not alone. I see and hear it everywhere. Ask anyone in their late twenties to name an MP from their lifetime they admire, and most will stare at you blankly. Plenty of us are engaged in politics, but without relish, voting in elections like we're choosing from the menu at Wimpy. When the expenses scandal broke in 2009, there were no marches, no protests, just a shrug of the shoulders and a look that said: why would we expect anything else? Since the early promise of Blair proved so misplaced, there hasn't been a single inspiring figure in the top tier of British politics. No wonder we idolise Obama from afar: how we long for a leader whose motives we can trust, no matter how they might fail.
The worst legacies of the Iraq War belong to the families of the soldiers and civilians from Iraq, Britain, America and everywhere else forced to make sacrifices for an illegal occupation. But another legacy, one harder to measure than body bags, is the way Tony Blair's hubris robbed a generation of their faith in politics.
I sometimes wonder how differently we'd feel today if Blair had listened. No doubt at the time he felt he was heeding the call of history, but he heard the wrong message. Ten years later, even when the last troop finally comes home, we'll all still be paying a price.
This article originally appeared at Huff Post