© Agence France-Presse / Getty ImagesOsama Bin Laden
The killing of Osama Bin Laden provided a moment of catharsis that had eluded America for a decade. Flag-draped crowds spontaneously gathered outside the White House and at Ground Zero in Manhattan, singing the national anthem. On television, Peter Bergen, an expert on al-Qaida and one of few people in the field to have actually met its leader, declared: "Killing bin Laden is the end of the war on terror."
The mood this morning is likely to be more sober, as Americans cast their minds back on past premature declarations of victory, in Afghanistan at the end of 2001 and at George W Bush's infamous "Mission Accomplished" triumphalism over Iraq in 2003.
The struggle against terrorism does not give itself easily to neat beginnings and endings. In one sense, the "war on terror" ended in March 2009 when the incoming Obama administration decided it was a counter-productive phrase in the first place, bringing America's enemies together rather than dividing them.
After being driven from Afghanistan in 2001, al-Qaida's response was to transform itself into a far looser global network that would be harder to destroy. In its most dilute form, al-Qaida is little more than a franchise that alienated groups around the world can sign up for, exchanging formal oaths of allegiance for the dread that the name inspires in their enemies.
