Nato allies have fought together in wars outside Europe before, notably in Iraq, but always in coalitions of the willing, not in an explicitly Nato operation. Afghanistan is, curiously, the first test of the North Atlantic Treaty Organisation on the battlefield, and no one familiar with Nato history will be surprised that, as Condoleezza Rice charitably put it, “it's bumpy and there is a lot of maturing that the Alliance is having to do”.

Beginning, she might have said, with facing the fact that “one for all and all for one” means all 26 members doing their fair share of the fighting. Nato has got over fiercer rows in the course of its 59-year-old history than the quarrel now raging over the reluctance of many Nato members to send troops to combat zones in Afghanistan. But none of those rows left Nato troops taking heavy casualties in tough conditions, with Nato members refusing point-blank to come to their aid. This does; and that is the cloud that Robert Gates, the US Defence Secretary, sees hanging over Nato's future as a military alliance.

Nato's cohesion - on which its credibility rested, and still rests - was never fire-tested during the Cold War. It ended without the alliance having had to fire a shot, but not without plenty of examples of Nato governments shooting themselves in the foot.

The alliance fractured so badly and so often that cynics observed that it was a miracle that Moscow ever took Nato seriously - it could not even take the step of basing medium-range US nuclear missiles in Europe without almost tearing itself apart. It was no miracle, of course. Moscow took Nato seriously because it took Washington seriously; Europeans in Nato resented, often doubted but ultimately counted on America's continued engagement; and the US, grumble though it might, and with reason, about unfair military “burden-sharing”, viewed Nato as its key force-multiplier in the containment of Soviet power. All deterrence has elements of a confidence trick and with history on its side, Nato pulled off the trick.

The alliance's mistake since 1989 has been to sideline and dilute its military role, as it expanded and reconfigured as a self-conciously political entity. The fault lies not with the new members, who still think of Nato as their strategic guarantee against Russian irredentism, but with Western European efforts to enfold Nato within the comforting web of “soft power” strategies, as an agent for “projecting stability”, putting out the odd bush fire and offering “robust peacekeeping”. A military alliance must be capable of more.

The troops for Afghanistan row must be had out in the open, not muffled, as Nato's Secretary-General would prefer. The issues are too important. Nato is at a crossroads. One signpost points towards some sort of a regional adjunct to the United Nations. The other beckons those few countries still able to defend the West.

To survive, Nato needs a redesign. A Nato MkII would divide the core military priorities from the ancillary political activities, in a two-tier organisation. It would no longer be tied by consensus decisions that have, in practice, become consensus plus opt-out. The first tier countries would provide, and decide when to deploy, properly equipped and operationally compatible fighting forces. Those unable or unwilling to fight would make up a Nato second tier, concentrating on aid and nation-building. The idea is anathema now. But Nato must march, and soon, to a different drum.