Only once in its history has Nato invoked Article 5 of its founding charter, which sees an attack on one member as an attack on them all. It was in the aftermath of the 9/11 terror attacks on the US. Nato troops were promptly deployed to overthrow the Taliban in Afghanistan.
They are still there and citizens of Nato countries are increasingly wondering why. The question hung over last week's summit in Strasbourg to mark the alliance's 60th birthday. One answer was provided by President Barack Obama in a speech on Friday. Nato is still at war, Mr Obama said, because the threat of terrorism is still real. To yield to the Taliban would put lives at risk far beyond Afghanistan's border.
Mr Obama's European counterparts appear convinced by that argument. Some, including Britain, have promised additional troops, although none will come close to matching the US contingent.
Logistically, America is holding back the Taliban with only auxiliary help from Europe. And that, for now, is the limit of US ambition. The strategy is to pin the insurgency back to a few areas, hoping to buy enough time for some kind of indigenous political process to set down roots in the rest of the country.
But what kind of process will that be? Public opinion rallied in support of the war not just because the Taliban regime gave safe haven to al-Qaida, but because it was oppressive, especially in its treatment of women.
Progress, however incremental, has been made in restoring basic civil rights to ordinary Afghans. That, in turn, has helped mitigate the sacrifices made as a consequence of military occupation, among Afghan civilians and Nato forces.
But there are signs that the limits of liberalisation have been reached. To shore up his position ahead of elections later this year, President Hamid Karzai recently backed a law giving men of the country's Shia minority total dominion over their wives, legalising child marriage, rape and incarceration inside the home. Even if, as seems likely, pressure from Nato leaders forces Mr Karzai to withdraw the law, questions remain over whether or not he can be trusted to uphold the country's post-Taliban constitution.
This reflects a fundamental problem for the western intervention. The strategy has been based so far on creating an Afghanistan that will most benefit local minorities - the more moderate, more literate and more tolerant populations - and women, who have very little power.
That has threatened the interests of the most violent and reactionary elements - generally men - and more specifically, rural Pashtun tribes who have traditionally ruled Afghanistan. No regime has survived long in Kabul without their support. They provided the most recruits for the original Taliban movement and are driving the current insurgency. That is why the counterinsurgency strategy has stalled. It is possible to make piecemeal progress, winning over individual factions. But large sections of the population see no self-interest in joining the "new" Afghanistan.
The west cannot support civil rights in Afghanistan without local support. But by definition the Afghans whose rights are most under threat are those who are least able to show support for the occupation. Yet they would be betrayed if Nato lost the will to fight on and the country descended into chaos.
So Mr Obama is right that the war must continue. But Nato must be clear that it is fighting to uphold the constitutional order of Afghanistan, not just whoever happens to oppose the Taliban at any moment. As long as a stable political process under that constitution remains a credible aspiration, alongside anti-terror operations, Nato has the chance to maintain legitimacy for its occupation. But once it is perceived to be backing just one set of oppressors against another, its mandate is effectively over.