During negotiations over the Washington Treaty, NATO's founding charter, the wording of Article 5 containing the collective-defence commitment was crafted to reassure European Allies of America's commitment to their security and ensure US Congressional and public acceptance of its terms. Ever since, the Allies have been forced to adapt Article 5's implementation to changing conditions within and outside the Alliance. It could be argued, in fact, that the way the commitment has been implemented has been at least as important as the Article's carefully chosen words.

UK Foreign Secretary Ernest Bevin is universally recognised as the father of the Washington Treaty and instigator of what became Article 5. In December 1947, following the failure of the four-party talks between France, the Soviet Union, the United Kingdom and the United States on the future of Germany, Bevin was convinced that the Western powers had to organise themselves to defend against Joseph Stalin's expansionist ambitions and Soviet military power. US President Harry S. Truman and Secretary of State George C. Marshall were initially unsure whether the United States should join any alliance to defend Europe. The US isolationist legacy and resistance to formal alliances was receding but remained well-represented in the US Congress, where any treaty would have to win two-thirds support in the US Senate.

Bevin nonetheless persisted. His historic speech to the House of Commons in January 1948 called for the creation of a Western European Union as the prelude to a transatlantic pact. On 17 March, the governments of Belgium, France, Luxembourg, the Netherlands and the United Kingdom signed the Brussels Treaty, a pact including a strong mutual-defence provision. The Treaty was immediately welcomed by President Truman and the action moved back to Washington.

Canadian, UK and US officials began meeting secretly in the Pentagon just five days later. Although the basic argument for a transatlantic pact was provided by Bevin and his colleagues, the US participants served as the intellectual and drafting mid-wives to the treaty, with the Canadians playing an important supporting role.

On 24 March, the US delegation submitted a memorandum endorsing the idea of a security pact for the North Atlantic area in which the "US government would regard an attack upon any of the Brussels Treaty Powers as an attack against the US, to be dealt with by the US in accordance with Article 51 of the United Nations Charter [which preserved the right of UN members to 'individual or collective self-defence']." Once the Truman administration had agreed to include this concept in the "Pentagon proposals", the next step was to bring Congress, the other US partner in the emerging "transatlantic bargain", into the deal.

In parallel with the secret tripartite discussions, the administration had begun working with a few key members of Congress to try to learn what kind of agreement might pass muster in the Senate. Senator Arthur Vandenberg, a reformed isolationist and Republican Chairman of the Senate Foreign Relations Committee, decided to incorporate the essence of the Pentagon proposals in a Senate resolution. Vandenberg not only agreed with the direction in which the talks had moved, but also wanted the Congress (and himself personally) to get some credit for the concept. The "Vandenberg Resolution" passed the Senate on 11 June, helping grease the skids for continued work on the Treaty.

In July 1948, the secret tripartite negotiations were succeeded by seven-power talks, as France and the Benelux countries joined the talks which now included all the Brussels Treaty powers, Canada and the United States. The French representative urged that instead of wasting time drafting a transatlantic treaty, the United States should immediately associate with the Brussels Treaty powers, send military assistance to France and others, and strengthen its military presence in Europe. Given that France was at this point not aware of the Pentagon proposals, and that the US government was becoming increasingly committed to a transatlantic defence arrangement, the French position was quite logical. With US forces deployed to the east of French borders, any attack by Soviet forces would instantly engage the United States, collective-defence commitment or not.

From the US perspective, however, the most important long-term question was the wording of the collective-defence commitment. Bevin and the other Europeans would have liked the new Treaty to replicate the Brussels Treaty's strong collective-defence commitment. This would commit Canada and the United States to automatic participation in defence against any attack on another member of the pact. US negotiators knew that such a provision would immediately be seen by the Congress as undercutting its constitutional powers for declaring war and would therefore have no chance of winning Senate consent. The US team preferred language similar to that of the 1947 Rio Treaty that the United States had signed with 21 other American states. The Rio Treaty provided for assistance in the event of an attack on any member but did not specify armed action. However, for the Europeans, leaving out the option of a military response would have voided the Treaty's value.

The eventual wording of Article 5, with its obvious ambiguities and compromises, nonetheless met the dual criteria of European reassurance and US political acceptance. The area covered by the commitment included an attack "in Europe or North America" and, in Article 6, also included the Algerian departments of France. It said any attack in this area "shall be considered an attack against them all..." Referring then to the legitimacy of such actions given by the UN Charter's Article 51, each Ally pledged to "assist the Party or Parties so attacked by taking forthwith, individually and in concert with other Parties, such action as it deems necessary including the use of armed force."

The Treaty, with this somewhat qualified but still meaningful pledge, was signed in Washington on 4 April 1949. The US Senate gave its consent to ratification on 21 July by a vote of 82-13.

Several Treaty opponents nonetheless had grasped an important reality. Senator Forrest Donnell argued that the "apparent freedom of choice on the part of the United States was illusory". During consideration of the treaty, Donnell asked, if the Soviet Union should invade Norway, "Do you think that we would be complying with our duty under this Article 5 if we should say, 'All we have to do is to take such action as we deem necessary? We think that just sending over 10 gallons of coal oil would be sufficient.'" NATO historian Lawrence S. Kaplan, writing about the leading Treaty opponent, Senator Robert Taft, notes: "There was no question in his mind that Article 5 entangled the nation in a war, whether or not Congress declared it before American forces actually took part."

In some respects, the Treaty and its collective-defence commitment could be seen as an audacious bluff, given the vulnerable condition of European countries in 1949 and widespread doubts that available military forces could stop a Soviet military campaign from reaching the English Channel. These circumstances underlay the French insistence that they needed military assistance from the United States more urgently than a transatlantic treaty. In any case, the bluff worked, perhaps because it held the promise of substantial US resources being committed to the defence of Europe, or perhaps because Stalin's strategy was to defeat European democracies from within via Moscow-supported communist parties before risking any more Russian lives on the battlefield.

If Article 5 were a bluff when the Treaty was signed in 1949, it became more substantial soon after. The Soviet Union provided plenty of incentive for Western action. The blockade of Berlin in 1948 and 1949 gave a sense of urgency to the drafting process. Soviet testing of a nuclear weapon in August 1949 spurred on US military assistance to the European Allies. North Korea's attack on South Korea in June 1950 solidified the US commitment to Article 5, leading the United States to send an additional four divisions to Europe.

As the United States deployed significant conventional forces and nuclear weapons in Europe, Article 5 became even more credible. Just as France had argued in the early stages of Treaty negotiations, and as Treaty opponents in the US Senate had feared, once the United States was fully engaged at the heart of the defence against a Soviet attack, Article 5 had much more meaning in practice than it had ever had in theory.

Throughout the Cold War, the Allies debated how best to implement Article 5. The initial US preference would have been to provide air power and supplemental ground forces to back up European forces on the ground. Europeans wanted US forces deployed forward in Europe to ensure that US interests would be engaged in the earliest stages of battle. Europeans, of course, never managed to provide the forces that would have been required to meet US preferences.

Motivated by the perceived need to limit US military spending as well as the failure of the European Allies to deploy sufficient conventional forces, President Dwight D. Eisenhower's administration insisted the Alliance adopt massive (nuclear) retaliation as the main instrument of collective defence, which it did in December 1954. When massive retaliation was undermined by the Soviet Union's acquisition of delivery systems capable of striking targets in the continental United States, the United States convinced the Allies to adopt a "flexible response" strategy, in which NATO would plan to respond to a Soviet attack with whatever conventional or nuclear forces were required to defeat the attack.

Flexible response never totally satisfied either the United States or the European Allies, and, as a consequence, the Alliance endured years of burden-sharing battles and debates over deployments of new nuclear weapons systems. However, in spite of all the strategic twists and turns, deterrence based on the Article 5 commitment held together until the Soviet Union fell apart.

At the end of the Cold War, NATO faced an entirely new set of questions, including Article 5's continued relevance. Without calling the commitment into question, the Allies began removing many of its implements. Most US nuclear weapons were withdrawn from Europe and the process of reducing US troop levels and re-orienting NATO strategy toward "new risks and dangers" was begun.

Many analysts, myself included, suggested in the mid-1990s that Article 5 was being put in NATO's "back pocket", available for use should it become necessary, and that non-Article 5 contingencies would increasingly preoccupy the Alliance and dominate its day-to-day political and military activities.

Although Article 5 was invoked in response to the 11 September 2001 terrorist attacks against the United States, NATO's activities are now largely rooted in the Washington Treaty's Article 4. This Article, which says that the Allies "will consult together whenever, in the opinion of any of them, the territorial integrity, political independence or security of any of the Parties is threatened", was included in the Treaty primarily as a result of European concerns over their remaining colonial interests. It promises nothing, as was clear in the Allied debate over the war in Iraq. But it invites the Treaty signatories to work together to defend their interests from threats emanating from any source, anywhere in the world.

That said, Article 5 still constitutes the heart and soul of the Alliance. It represents a solid commitment to cooperate, and it provides a continuing rationale for NATO's Integrated Military Structure, without which the Alliance would not have the capacity to conduct non-Article-5 missions. Perhaps those prescient European and US officials who composed this seemingly timeless document would be surprised by how effectively it has served as a continuing basis for Euro-Atlantic security cooperation.