ITV News Security Editor Rohit Kachroo is in the US where the country is mourning its troops and planning its retaliation
It is 20 years, almost to the day, since a US president promised to “hunt down” terrorists in Afghanistan who killed Americans.
And on Thursday night, as President Joe Biden addressed the nation from the White House, his voice breaking, he echoed the words of George W Bush following the 9/11 attacks as he vowed, “We will not forgive, we will not forget”.
But this moment is so different.
What is the risk of further attacks in Kabul? Rohit Kachroo has the latest
In many ways, America’s longest war is ending as it began. But Islamic State-Khorasan Province (ISIS-K), the local offshoot of Islamic State group which is thought to have been behind Thursday's Kabul airport terror attack, does not pose the same threat to the United States that al Qaeda did in 9/11.
A report by the United Nations Security Council released in June said the network “remains diminished from its zenith”.
However, Mr Biden appears to have conceded that the airport attack might represent a change in the terrorist threat to the United States. (During a television interview just a week ago, he downplayed the risk to the US from violent extremists in Afghanistan).
ISIS-K has long been defined by its loathing of the Taliban, a feeling which is mutual. But on Thursday, the US was its target too.
The Islamic State is blurring the lines between its enemies, portraying the Taliban as sell-outs to its new partners in the White House - it hopes to recruit Taliban supporters disaffected by its change of tone since trying to form a government.
So what happens now? Mr Biden has asked the US military to explore options for strikes against ISIS-K.
But then what? After all, former president Donald Trump dropped the so-called "mother of all bombs", the most powerful bomb there is, on an ISIS-K hideout in 2017, which harmed but didn’t finish the group.
Mr Biden boasts of the “over-the-horizon“ counterterrorism capability available to his intelligence agencies.
But with far fewer eyes and ears of his own on the ground, American reliance on the Taliban is likely to increase.
A security partnership, which was unthinkable a fortnight ago, will inevitably expand as both sides see the value of each other’s intelligence in the fight against ISIS-K.
But that, in turn, might offer their mutual enemy a propaganda victory.
The United States’ search for retaliation might not be simple or short.