Nato was established 60 years ago today to provide collective security for its members. During the Cold War, the alliance proved to be not only an effective means of defending Western Europe from totalitarian aggression. It also became the most successful liberation movement in history, by securing the space of freedom. But since the collapse of communism, the alliance has lost an adversary and not found a role.

The Strasbourg Nato summit provides an opportunity to advance the cause of collective defence in a hostile international order. But the rhetoric has outstripped the evidence of practical commitment. Nowhere is this divergence clearer and more damaging than in defending constitutional government in Afghanistan against the resurgent Taleban. The British Government must make clear its own commitment to the defence and expansion of liberal values. The signals are as yet mixed.

The world’s inattention to Afghanistan in the first place allowed the country to be, in effect, taken over by a transplanted Islamist group that plotted the 9/11 atrocities. Nato and its International Security Assistance Force (Isaf) have had to become more effective in counter-insurgency. Nato needs more troops in order to establish full military control. Mr Brown announced yesterday that several hundred more troops would be committed to Afghanistan to provide security before the presidential election in August. This is far fewer than the 2,000 that General Sir Richard Dannatt, the outgoing head of the Army, has said that Britain could send. And British troops, who provide the second-largest commitment to Nato’s Afghan mission, have already shown courage in that task.

But the announcement is hardly full-throated. It comes in a week when British Forces have been withdrawing from Iraq and could send 2,000 combat troops to join the US surge of 21,000 reinforcements now being sent to Afghanistan. The mission of British Forces must be adequately supported, especially financially. It is a test of the special relationship that Mr Brown espouses. There is more to statesmanship than the international financial architecture. There is the preservation of liberty, which is Nato’s cause.

Nato leaders intend to initiate a review of strategic doctrine, which could be adopted at the Nato summit next year in Lisbon. This would certainly be timely. The Nato allies have sought since the Washington summit of 1999 to engineer a new Nato capable of responding flexibly to threats beyond Europe. The dangers of conflagration in Europe have been much reduced since the collapse of communism. Emerging threats such as nuclear proliferation to rogue states and the spread of Islamist terrorism have become far more pressing.

Nato needs a post-9/11 security doctrine to take account of them. It is extraordinary that it does not yet have one. The transformation in the fortunes of Iraq after the consistent application of a new approach under the command of General David Petraeus demonstrates that doctrine matters. Hard thinking about the nature of threats, and how best to anticipate and counter them, will preserve liberty and save lives.

But even so, the inchoate state of Nato thinking might be compensated for by a clear commitment of troops and diplomatic urgency. Yet even with known problems — let alone with what a former US defence secretary termed known unknowns — Nato’s governments lack unity and display equivocation. President Obama noted yesterday that al-Qaeda posed a greater danger to Europe than to the US. He rightly urged a better use of Nato resources in Afghanistan. It is a message that European governments, not least Gordon Brown’s, should heed.