In his State of the Union address to the American people earlier this year, Barack Obama declared that he was "confident" of achieving "our objective of defeating the core of al-Qaeda".
Although he acknowledged the need to pursue the "remnants" of the terrorist group and its affiliates, the overall message was clear – al-Qaeda was badly degraded, the tides of war were receding and the US was winning this fight that was no longer even officially a war.
The Boston bombings would appear to present a fundamental challenge to that assessment and once again bring the nagging uncertainty of terrorism back on to the American main street.
It is too soon to be absolutely sure the attacks were motivated by jihadist ideology, but the Islamic videos on the website of the older of the two Tsarnaev brothers point very firmly in that direction.
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They bring home the complexity of the global Islamist threat and the fact that it cannot be confined to wars in distant lands, or fought at arm's length using drones, as the Obama administration has quietly yet insistently led America to believe.
Mr Obama and his intelligence community know the threat from al-Qaeda affiliates, but have chosen to downplay it to the US public.
Even when that fight does directly touch on American lives, as it did last September when the US ambassador to Libya was murdered in Benghazi by an al-Qaeda linked group, the administration appears at pains to deny the connection.
Indeed, next week, America's transportation authority is to relax rules on carrying knives on planes for the first time since the September 11 attacks.
But as many counter-terrorism experts have been saying – their voices often drowned out or ignored in favour of the pleasing simplicity of the Obama administration's narrative – the threat from al-Qaeda is too amorphous and shifting to ever have been discounted.
"They've fallen into the same trap that the Bush administration did early on," says Tom Jocelyn, a senior fellow at the Foundation for Defense of Democracies think tank who tracks the movements of high-value al-Qaeda targets.
"They define al-Qaeda as a hierarchical terrorist organisation such that if you kill 'x' number of leaders then the whole thing falls apart."
But the early information on the Tsarnaev brothers – born in Kyrgyzstan to a Chechen family, but living in the US for up to a decade – points to just how blurred, in reality, the distinctions between al-Qaeda and its affiliates can become.
"It's a hybrid thing, that's the problem," says Aaron Zelin, a fellow at the Washington Institute for Near East Policy who has written extensively about the decentralisation of al-Qaeda. "It's a unique threat, there's nothing like it and that's why people have a hard time grasping what it is."
Looked at that way, Mr Obama's "confidence" – and that of the American public – is likely to be badly shaken by what is emerging from Boston.
