BBC diplomatic correspondent
Defining exactly "What is Nato for?" has been a problem ever since the end of the Cold war.
Balkans became a testing ground for the new Nato
After all, the Alliance was created to counter the Soviet threat.
So when the Soviet Union collapsed and its former client states rushed to rejoin a united Europe, Nato triumph mingled with confusion.
Operations in Europe's backyard, first in Bosnia and then Kosovo, offered one solution.
But Kosovo was also contentious: it was Nato's first ever offensive military operation, an encroachment on another country's sovereignty in the name of humanitarian protection. It temporarily rocked a fragile new relationship with Russia.
Arguments behind closed doors at Nato headquarters during the bombing campaign left the Americans determined never again to go to war by committee, especially when the Alliance was expanding rapidly.
So when 11 September came, it is hardly surprising that Nato's dilemma sharpened. For the first time ever, it invoked Article Five of its Charter, signalling its readiness to take military action to aid a fellow member.
An historic moment, but the Bush Administration had other ideas. The attack on the Taleban in Afghanistan was deliberately a US-led, adhoc "coalition of the willing".
Nato provided some air and naval backup, but the American response looked like a snub, and a warning that in a world of global, unconventional threats, Nato needed to reinvent itself... or risk becoming irrelevant.
The Iraq invasion only intensified the crisis, splitting the Alliance. Hostility towards US policy reignited talk among some members of a separate European defence capability that might one day undermine Nato's raison d'etre.
It hasn't all been down hill though. Throughout the 1990s an imaginative Partnership for Peace programme meant Nato forged new informal links, including joint military exercises, as far afield as Central Asia.
A new Nato-Russia Council encouraged collaboration in various ways - including agreements on rescue-at-sea and terrorism. The early insertion of Nato troops in Macedonia helped avoid a new Balkan conflagration. And in Afghanistan Nato forces showed the Alliance could adapt to new tasks well beyond Europe.
Indeed, Nato now sells itself as broad security alliance, a force for stability in Europe, as well as a tool box of a highly trained forces, ready for new challenges.
That's all very well, but let's not forget that continuing disagreements inside the Alliance have limited its role in post war Iraq to training.
And though it is true that new members like Estonia certainly see Nato as an essential protector against potentially hostile neighbours, that poses a new problem: the fuelling of Russian suspicion.
Fifteen years ago, Nato's existential challenge was how to cope with the demise of its old enemy: the USSR.
The irony is that 15 years on, with former Soviet client states like Ukraine, Romania and Georgia all electing pro-Western leaders, it is once again relations with Moscow that could prove the most important and the most problematic.