Millions watched in horror as the 9/11 attacks unfolded 25 years ago, killing 2,977 people in the worst attack on American soil since Pearl Harbour 60 years before.
It sent a high-voltage shock into a largely ignorant world that suddenly became aware of the al-Qaeda threat but intelligence agencies were caught on the hop too.
Head of MI5 Ken McCallum recalled in today’s threat update speech that he was five years into his career when Osama bin Laden’s terrorists struck the USA.
Sixty-seven Brits were among the victims but the spin-off attacks across the world, all the way into the streets of Britain claimed thousands more.
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It sparked wars in Iraq and Afghanistan, hundreds of billions of pounds spent on securing the world’s transport hubs and changed the world forever.
Few people have experienced the high-intensity battle in the shadows against terrorism at the level as Mr McCallum, or even the depth of first-hand experience he has of MI5 investigations.
He has led British counter-terrorism and the fight against espionage for six years but has fought both for three decades as an officer within MI5.
In his memory lies the sudden intensity with which 9/11 reshaped MI5, the flood of incoming new officers, language experts, recruitment of “agent” insiders to infiltrate jihad.
That shift from struggling against Russian and Chinese spies, terrorism in Northern Ireland rebuilt MI5 with bigger budgets and a major change in emphasis towards fighting Islamist terror.
Sources who were around at the time say the change up in pace was almost at break-neck speed as MI5 caught up with the emerging threat of al-Qaeda and its offshoot Islamic State.
Mr McCallum warned “we’re there again,” as he acknowledged yet again that MI5 is undergoing a major shift are far more significant than many people might believe.
He admits our domestic spies are undergoing a “seismic” change to react to the emerging and ever-changing threat to the UK from a number of enemies.
Whilst the Islamist threat still exists, we have to add to that the threat of subversion and violence from the Russian and Iranian states.
Artificial intelligence, he warns, could assist bad actors in moving bio-warfare along, the toxicity of internet messaging can be harnessed to incite division in UK society.
Young people are being radicalised, turned against sections of society in ways that were unimaginable a quarter of a century ago.
Banking systems, underwater cables, critical infrastructure, our health systems are all under threat from a daily barrage of sub-threshold for war attacks.
The 9/11 attacks were a shocking, heartbreaking atrocity that has echoed through the past 25 years bitterly and never seems to go away as Islamist terror grew at a terrifying pace.
Any Brit who does not already know that there is the risk of a terror attack or aggression from Britain’s enemies must have been asleep for the past 25 years.
But Mr McCallum’s words hold huge meaning beyond the usual warning about bombs, bullets, knives and possible car-ramming attacks. They go without saying these days.
He was telling us what many already suspect – that every aspect of UK society is being assailed and exploited to drive a wedge between different sections of our communities.
And the head of MI5 knows where this could lead.
His appeal for the British public to “think” about how we are being attacked and to accept that we all play a role in not accepting division and hatred is just the beginning.
It is highly likely that in the coming months and years we will remember his words and recall how certain public figures and members of the public failed to recognise it for what it was.
This was a call to arms, not to physically engage with an enemy, but to use the weapons of persuasion and reason to combat what states like Russia want more than anything. That is to spin-up hatred among us, to promote racism and resentment, to hugely, and disproportionately inflate issues such as immigration to create division in society.
Not only are MI5’s officers physically and mentally fighting terrorism in the shadows, they are also battling against the UK’s enemies interfering with the minds of our neighbours.
And that may be the toughest battle of them all.