On 11 September 2001, the international terrorist organisation known as al Qaida achieved something that the Soviet Union never attempted. It killed large numbers of Americans, together with many non-Americans, on US soil. The carnage and death toll inflicted on that day were greater than that inflicted 60 years earlier during the attack on Pearl Harbor, the event that brought the United States into the Second World War. And its impact on both the wider security environment and NATO can hardly be over-estimated.

The very next day, the North Atlantic Council, NATO's highest decision-making body, decided that: "If it were determined that this attack was directed from abroad against the United States" then it would be regarded as an action covered by Article 5 of the Washington Treaty, the most important clause of the Alliance's founding charter. After briefings by US officials to NATO member states on 2 October, the condition relating to externality of attack was deemed to have been satisfied. In this way, NATO's so-called "commitment clause" came fully into effect.

The irony of NATO's decision was immediately obvious. The Berlin Wall had been breached almost 12 years earlier on 9 November 1989 (11/9) and NATO had won the Cold War without needing to invoke Article 5, the political and military "heart" of its founding charter, or even firing a single shot in anger. Moreover, although the clause was clearly envisaged by the Washington Treaty's signatories as a mechanism by which the United States would come to the assistance of its European Allies, it was the European Allies who were offering Washington their support.

Given the enormity of the events of 9/11, it is no exaggeration to say that they brought NATO's post-Cold War adaptation to an abrupt end. If, therefore, the period between the 11/9 fall of the Berlin Wall and the 9/11 terrorist attacks forms a distinct second phase of the Alliance's history after four decades of Cold War, then the symbolic significance of the invocation of Article 5 heralded the beginning of a third post-post Cold War phase, the ramifications of which are still emerging five years on.

Though clearly the invocation of Article 5 was a historical milestone, some analysts have sought to downplay its significance and even the importance of Article 5 itself. Citing the careful wording of the original text, they argue that the commitment clause has minimal real value and was little more than a smoke screen.

On the one hand, Article 5 stipulates that an attack on one shall be deemed equivalent to an attack on all, that Allies are obliged to respond, and that military force is an option. On the other hand, it also states that any given Ally "will assist the Party or Parties so attacked by taking forthwith. such action as it deems necessary". However, to understand the force and significance of the clause and the Alliance itself, the motivations of the original framers must also be taken into account.

The Washington Treaty, which is remarkable in comparison to similar documents in its brevity and clarity, was drafted as a political statement as well as a legally binding document. As such, it was a compromise between two existing models for collective defence, namely the Rio Pact of 1947 and the Brussels Treaty of 1948. The former, agreed among American states, pledged signatories to "assist in meeting the attack" against a fellow signatory; the latter, agreed among Western European countries, stated that members must "afford the Party so attacked all the military and other aid and assistance in their power".

The framers of NATO's founding charter understood the first to be too weak a formulation and the second to be too all-encompassing, given that certain founding members, such as Iceland, could not reasonably be expected to provide a military response to an attack, yet by dint of geographic location, or for other reasons, could make their own contribution to collective defence. As a result, the obligation was automatic but not restricted to a military response.

In this way, NATO also sent a political message to the world. As an alliance, it respected the will of its member states and allowed them the freedom to choose the nature of the response best suited to their own situation. This characteristic of NATO would be cast in sharp relief six years later when its adversary created its own formal alliance, the Warsaw Treaty Organisation, an involuntary alliance in which the will of the members was not a factor and where all interests were superseded by those of Moscow.

In addition to considering the political nuances of the original intent behind Article 5, the perception of the military threat among both the Washington Treaty's framers and its signatory heads of state and government needs to be taken into account. At the time, Western Europe appeared massively outgunned by the Soviet Union and therefore vulnerable to a blitzkrieg-style invasion. Subsequently, the scenario was one in which a weak Europe would be assisted upon attack militarily by the United States. Should the Soviet Union invade, the world's only nuclear superpower would come to the rescue.

What the Alliance's founding fathers could never have predicted was the scenario in which Article 5 would be invoked. On 9/11, the Soviet Union and Warsaw Pact no longer existed. Moreover, it was not Europe that was conventionally attacked by a nation state and its allies, but the United States by a non-state actor using wholly unconventional means. NATO's founding charter had been overtaken by events.

What, therefore, were the immediate consequences of Article 5's invocation and how has NATO coped with the fundamental change in its operational responsibilities?

While the Alliance is today extremely active in Afghanistan, where it runs the International Security Assistance Force (ISAF), Washington chose to operate outside the NATO framework in ousting the Taliban and al Qaida from Afghanistan, despite the invocation of Article 5. Indeed, to make things clear, when Richard Armitage, then US Deputy Secretary of State, came to NATO Headquarters days after the 9/11 attacks, he stated bluntly: "I didn't.come here to ask for anything."

The US decision effectively to do without NATO support reflected US perceptions of the Alliance's performance during the 1999 Kosovo campaign, the then limits of NATO's anti-terrorism capabilities, and a desire to avoid future political problems. Rightly or wrongly, NATO was associated with "targeting by committee", which was not deemed to be a sufficiently efficient mode of operation. Although the United States recognised that NATO had come a long way since the Cold War, the Alliance was clearly not configured to execute counter-terrorist operations in Central Asia. Moreover, Washington did not wish to have its hands tied by the need for consensus in the North Atlantic Council in the event of future campaigns, such as the invasion of Iraq.

Some analysts have argued that the European members of NATO failed to make a more robust response to the terrorist threat because of the absence of shared threat perception among Allies - a loss of what Phillip Gordon of Washington's Brookings Institution has called the "glue" that held the transatlantic community together for so long. However, this is not necessarily so.

Despite deep political differences over the Iraq campaign, the US National Security Strategy and the EU Security Strategy are similar documents and security professionals whose job it is to assess the threat to their countries, whether in Berlin, Paris or Washington, are largely agreed that the looming menace is extremist Islamist terrorism. Moreover, following the 2005 attacks in London and Madrid, it is clear that Europe is no longer at peace.

Despite Washington's decision to go it largely alone in Afghanistan, 14 of the then 19 NATO Allies contributed forces to the campaign to oust the Taliban and al Qaida in 2001. Moreover, the invocation of Article 5 has been fundamental to the Alliance's retooling in the intervening period to equip it with the capabilities to take on operations such as ISAF. In effect, it set in train NATO's ongoing post-post-Cold War transformation. In the process, the Alliance has built new command structures, launched various capabilities initiatives, developed some terrorism-related competencies and created the NATO Response Force. It has also moved well beyond the Euro-Atlantic area with operations and missions in Iraq, Pakistan and Sudan, in addition to Afghanistan.

NATO is not and never has been a club of homogeneous states. Rather it has traditionally provided and continues to provide different things to its different members. For many and in particular the new Allies, Article 5 remains a cornerstone of the Alliance. For others, the clause retains a greater political importance. And others see the Alliance's value in practical terms in its new out-of-area missions and operations, which are not a part of the traditional menu of war-fighting skills.

Finally, there are those who believe that NATO has demonstrated that it can adapt over time to new challenges, and that in time Article 5 may come to be understood as having direct relevance not to scenarios of invasion, but to the ways Allies collectively combat the scourge of international terrorism. As a result, while EU watchers have for years spoken of the possibility of a multi-speed European Union, NATO has already created the reality of a multi-speed alliance, that is one able to serve many purposes to cater to the diverse needs of its many members.

The debate over whether NATO remains a collective-defence organisation or whether it is turning into an alliance for collective security is largely academic. The Alliance satisfies both needs and will continue to do so for some time. Moreover, as such, it possesses capabilities that no other international organisation possesses. As for Article 5's historic invocation, we may do well to agree with the assessment of former Secretary General Lord Robertson, namely that: "It is still too early to say what the decision on Article 5 will mean in practical terms for the immediate future."

The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area. Any such armed attack and all measures taken as a result thereof shall immediately be reported to the Security Council. Such measures shall be terminated when the Security Council has taken the measures necessary to restore and maintain international peace and security.