What is the North Atlantic Treaty Organisation, or Nato, really for? It's a question that has dogged the alliance ever since the ending of the Cold War.
The real battle for the alliance's future may only just be starting
Confronting the Soviet Union gave Nato a clearly defined task - one on which all allies could agree.
With its former enemy gone, Nato was initially in denial. The alliance was as relevant as ever, said the press statements - and to prove it Nato expanded eastwards, taking in several former Warsaw Pact countries and even fragments of the old Soviet Union itself.
Peacekeeping in Bosnia and Nato's air campaign against Serbian forces in Kosovo seemed to add weight to the argument.
The Cold War may be dead, but Nato would live on. But then came Afghanistan and Iraq.
Wounds and divisions
In the former, Nato opted to invoke its most basic security pledge - to stand by the United States. Nato radar and command aircraft were despatched to patrol US skies.
But as far as the Afghan war itself was concerned, the call never came. Washington preferred to fight this one alone, in concert with a small number of hand-picked allies with specialised capabilities.
Over Iraq the alliance was so badly divided that some pundits openly questioned Nato's survival.
Afghanistan is seen as an example of Nato ambitions unfulfilled
So here we all are in Istanbul for another Nato summit. The wounds caused by the divisions over Iraq are healing, but that question - "What is Nato for?" - is still very much unanswered.
But put the question another way and a potential answer or answers become clearer.
If Nato did not exist what would be lost? In diplomatic terms, for all its faults, the alliance remains an important forum within which to discuss common transatlantic security concerns.
There may no longer be unanimity. But if used properly, Nato can still be an organisation which channels and contains differences and establishes common positions.
The near-rupture over Iraq was not fated but the result of circumstance, some pretty poor diplomacy, and a fair measure of posturing, and not just by one country alone.
Ambitions 'a stretch'
In practical terms, Nato remains the only effective military sub-contractor in the world capable of mounting a range of missions, from peacekeeping to peace enforcement.
The new secretary general wants to see a new predictability: a certainty that commitments, when made, will be honoured
Even when engaged in operations outside Nato, the alliance's common training and procedures ensure that a variety of armed forces can work effectively alongside each other.
Nato's geographical concerns are shifting. It is expanding operations in Afghanistan; proposing to take some as yet undefined training role in Iraq; and looking to establish new partnerships with its neighbours across the Mediterranean.
But there is one huge weakness, clearly identified by the alliance's new Secretary General Jaap de Hoop Scheffer.
Nato's ambitions far outstretch its capacity to deliver. Afghanistan is the best example of this.
Nato has struggled to find a handful of resources for even a minimal expansion of its operations there.
Member governments talk tough when it comes to taking on commitments, but resort to excuses when, for example, just a small number of helicopters or transport aircraft are required.
Battle just begun
The new secretary general wants to see a new predictability: a certainty that commitments, when made, will be honoured.
Nato's tradition that those who contribute to operations pay their own costs also creates a fundamental unfairness, with the financial burden falling disproportionately on countries who are willing to pitch in.
This too needs to be changed. The debate will begin in Istanbul. But these are major issues that go to the heart of how Nato is run.
So the real battle for the alliance's future may only just be getting under way.