Edgar Buckley describes how NATO invoked Article 5 on 12 September 2001, 24 hours after the terrorist attacks against the United States.

Dawn of a new security era : At the North Atlantic Council meeting immediately following the 9/11 attacks, all delegations spoke in favour of the strongest possible response (© Reuters)

I was chairing a meeting of NATO's Policy Coordination Group when news of the first plane hitting the World Trade Center came through. The executive secretary passed me a message to this effect. I read it and then read it out loud to the meeting. The US delegates looked shocked and disbelieving. We all were.

A few minutes later, the news arrived of the second plane hitting the second tower and I was told that the Secretary General was considering evacuating NATO Headquarters in case similar attacks were planned in Europe. I read this information out as well. Ken Huffman was leading the US team and asked for the floor. He announced that US Ambassador Nick Burns had already decided that US personnel should leave the building. I adjourned the meeting and went to the Secretary General's office.

The scene there was one of confusion. There were reports of unidentified or non-responding aircraft flying towards Brussels, though it was not clear how reliable the information was. The decision had indeed been taken to evacuate all non-essential personnel. Should the North Atlantic Council meet and if so where? It was decided that the Ambassadors would meet informally that evening in the Secretary General's office. There would be a formal Council meeting the next day.

In the Secretary General's outer office, I met Burns and Canadian Ambassador David Wright. Ambassador Burns was talking of the likely casualty figures - many thousands and probably the largest toll in a single day since the battle of Antietam during the American Civil War. Ambassador Wright, who was also dean of the Council, assured him of the support of all the Allies. "Hell, this is an Alliance," he said. "We've got Article 5."

That was the first reference to Article 5 I heard that day and it struck an immediate chord. As Assistant Secretary General for Defence Planning and Operations, I knew we had a job to do, even though no one had asked us. Particularly in an emergency, the Council looks to the Secretary General to provide leadership and proposals. In contrast to many other international organisations, responsibility for drafting documents and resolutions in NATO lies with the International Staff. We needed to prepare the advice and recommendations which the Secretary General would deliver to the Council in the morning.

My mind turned to a possible decision sheet or statement. The Secretary General, on his own initiative, had already issued a short condemnation of the attacks, but that was not enough. What could the Council itself add?

There was little to guide us. There had been hardly any discussion of terrorism at NATO up to that point. There was no clear policy, as far as I knew, on the use of NATO assets in response to terrorist attacks. There had been no consultation with delegations about what had gone on that day. We had not even discussed the way ahead or options with the Secretary General or his Private Office. There had been no "steer" from any capital.

I first raised the possibility of a statement invoking Article 5 at a meeting in Günter Altenburg's office in the early evening. As Assistant Secretary General for Political Affairs, Günter would also be involved in advising the Secretary General and we needed to be united. I took the text of the Washington Treaty with me.

Ted Whiteside, head of NATO's Weapons of Mass Destruction Centre, was also present and questioned whether the attack that day had been "armed": Article 5 contemplated an "armed attack" against any NATO Ally, but was an aircraft a weapon? We also discussed how to distinguish what had happened that day from "normal" terrorism, such as practised by the IRA, ETA or the PKK. The discussion was useful but inconclusive. I went back to my office and called in Holger Pfeiffer, my deputy, and Steve Sturm, head of defence policy.

We went through all the issues on our own. We agreed that there had indeed been an armed attack. The aircraft had been used as missiles. As for distinguishing such an attack from "normal" terrorism, we selected two criteria - the scale and external direction. The scale was important, we felt, because the Washington Treaty had been written to deal with threats to peace and security in the North Atlantic area, which implied a high threshold of the use or impact of force. External direction was important because it was clear that the Allies did not regard attacks by internal terrorist organisations - such as in Belfast or Oklahoma City - as falling under the Treaty. There was, of course, another way to distinguish one terrorist attack from another, namely by an ad hoc decision of the North Atlantic Council. If the Allies were to determine that an attack met the criteria for a response under Article 5, that would be conclusive.

We quickly satisfied ourselves on these grounds that there was a good case for declaring that the attacks had triggered the Washington Treaty's collective-defence provisions. The next step was to research what supporting policy statements there might be for such a determination in earlier NATO documents and communiqués, because referring to existing agreed language is an important step in facilitating consensus. I asked Steve Sturm to look at the 1999 Washington Summit declaration and Strategic Concept in particular to see what from those documents could be used to strengthen our approach, and to check what other policy statements had been made about terrorism. I also asked him to produce a first draft based on our discussion.

An hour later, we met again and went through the draft together. We inserted a conditional "if" clause to deal with the uncertainty over who had directed the attacks: "If it is determined that this attack was directed from abroad against the United States, it shall be regarded as an action covered by Article 5 of the Washington Treaty." We referred to the condemnation of terrorism by the heads of state and government at the Washington Summit and their statement that they were determined to combat terrorism in accordance with their commitments to one another. We finished our work and had a typed draft ready to present to the Secretary General early the next morning.

I went to the Private Office at around 7:30 in the morning with the draft statement in my hand. Lord Robertson was standing in the doorway to his office. I gave it to him and said we had prepared it in case the Council wanted to make the strongest possible statement of its support. He read it, liked it and passed it to Damon Wilson, the US deputy director of the Private Office, with the instruction that it should be sent immediately to the US authorities for their reaction. The Council was due to meet a few hours later.

The US response was quick. I subsequently learned that after consulting with his deputy, Toria Nuland, Burns had passed the text to Secretary of State Colin Powell with his recommendation to support it. Powell quickly authorised Burns to do so and in parallel consulted President George W. Bush. By the time the Council met, President Bush had signalled his support too.

At the Council, which had a very restricted attendance, all delegations spoke in favour of the strongest possible NATO response and almost all were ready to approve the draft statement, which had been circulated by the Secretary General in advance. A small group of nations asked, however, for legal clarification as to the effect of invoking Article 5. They had two main concerns. First, they wanted to ensure that their sovereign decision-making rights would not be affected as regards the nature, scale and timing of actions deemed necessary to restore peace and security - in other words they wanted it to be clear that each Ally would deem for itself what was "necessary". Second, they wanted to ensure that any collective action taken by the Alliance, for example military action by NATO forces, would not be launched without specific additional consultation and decision in the Council.

Confident that these points could be quickly dealt with by NATO's legal adviser, Baldwin De Vidts, the Secretary General adjourned the meeting until later in the day. When the Council reassembled, it did so with De Vidts' reassuring memorandum in front of them. In effect, he concluded that it was up to each Ally to judge for itself what action needed to be taken, although such action should be appropriate to the scale of the attack, the means of each country and the steps necessary to restore peace and security. On the question of collective response, he said it was obvious that collective consultation would be necessary. With these clarifications, and after a short discussion, the Council unanimously agreed the draft statement as circulated. It was issued that evening and, in accordance with the Washington Treaty, Lord Robertson informed UN Secretary General Kofi Annan in parallel.

There had been no significant change to the wording of the statement from the time it left my office to the time it was issued less than 16 hours later.

How did we feel after the decision was taken? In truth, we were convinced that it was the only right and proper course. We were ready to implement it with the support of all Allies. And we felt just a little elated that the Allies had reacted so promptly.

Politically and publicly, NATO's action had a dramatic effect. Lying in bed the next weekend, I heard Alistair Cooke in his Letter from America reflecting on the remarkable heroism of the rescue workers and ordinary citizens caught up in the aftermath of the attacks. It was moving stuff. Then, at the end of his broadcast, he turned unexpectedly to NATO. He said that what the Alliance had done immediately after the attacks was the "one small note in this whole monstrous story that can be called heartening".

In the intervening five years, I have heard frequent criticism of the decision to invoke Article 5. I have, for example, heard people say that we were unwise to commit ourselves to a course of action which was not fully implemented and which turned out to be unwanted by the United States.

I was present in the Council two weeks after NATO invoked Article 5 when then US Deputy Secretary of Defence Paul Wolfowitz set out his post-9/11 doctrine to the effect that the mission determines the coalition. This was, in my opinion, a fundamental misjudgement about the nature of the Alliance that devalued the importance of strategic solidarity. As a result, I share the frustration of those who believe that the United States could have done more to engage the Alliance in its efforts against the Taliban and al Qaida.

However, I reject criticism of the decision to invoke Article 5. Following the lifting of the "if" clause on 2 October, the Allies - collectively and individually - did everything that the United States asked of them and were ready to do more. Moreover, in the intervening years Washington has come increasingly to recognise the importance of NATO and alliances in general and is learning lessons from its experiences in Afghanistan and Iraq.

Today, NATO is extremely active in Afghanistan. The United States is more convinced than ever of the need to secure international support for its actions against terrorism. And NATO is transforming its political and military structures and strategies to deal more effectively with the real security threats we face. Such developments would not have been possible had the Allies not stood side by side at the outset of this new security era.