President Obama and the leaders of the world’s most successful alliance—one that deterred nuclear war and kept the peace in Europe after centuries of conflict—gather in Chicago this weekend to talk about the future.

The North Atlantic Treaty Organization, NATO, has seen more fighting since the end of the Cold War than it did during the 50 years after the end of the Second World War. NATO allies have fought together in Bosnia, Serbia, and Kosovo, then in Afghanistan and most recently in Libya.

Yet in many ways the most remarkable achievement of the NATO alliance has been its contribution to peace, security and stability through the modernization and professionalization of the militaries of NATO countries and NATO aspirants. Gradually, NATO membership —or the prospect of it—lead the armed forces of 28 countries to evolve toward greater interoperability, improved capabilities and respect for civilian authority.

Article 10 of the North Atlantic Treaty which established NATO provides a process for adding new members to the alliance. The North Atlantic Council, made up of the civilian leaders of NATO-member governments, oversees the process. New members can be added only by unanimous consent because the core commitment of NATO members is that an attack against one is an attack against all; this bedrock principle of the alliance would not hold if a member did not have full confidence that each of the others would come to their aid in the event of war.

Altogether, NATO has added members six times. In 1952, Greece and Turkey became NATO members as a tangible affirmation of the Truman doctrine. In 1955, West Germany was admitted over the objections of the Soviet Union, which occupied East Germany; it marked the formal recognition that the four zones of Germany occupied by the victorious powers in the Second World War had become de facto two separate countries, linked to the opposing camps of the Cold War. The border between East and West Germany remained NATO’s front line until German reunification brought all of the unified country into NATO in 1990.

Spain was admitted to NATO membership in 1982, the third major addition to the alliance, in recognition that the Spanish dictatorship of Francisco Franco had ended and the country had again embraced democracy. It wasn’t until the Cold War ended that NATO expansion would again be considered.

Nearly 20 years ago at the NATO summit in Brussels in 1993, the North Atlantic Council inaugurated a new program it called the Partnership for Peace (PfP). Countries that took part in PfP hosted military exchanges with NATO member militaries, shared information on training, equipment, and tactics to foster trust and reduce potential misunderstanding that could lead to conflict. Former Warsaw Pact member countries and many of the former republics of the Soviet Union were invited to participate in NATO’s PfP program. So too were former neutral or nonaligned countries in Europe such as Austria, Sweden, and Switzerland.

The fourth addition to NATO ranks occurred in 1999 at a summit in Washington. The Czech Republic, Hungary and Romania graduated from PfP participation to alliance membership. This created the hope (if not the expectation) that joining the PfP was a step toward NATO membership.

Based on the experience of adding these three former Warsaw Pact militaries, the NATO leaders formally established a process by which prospective members would negotiate a series of commitments as part of a Membership Action Plan which included (but was not limited to): a willingness to settle international, ethnic or external territorial disputes by peaceful means; commitment to the rule of law and human rights, and democratic control of armed forces; the ability to contribute to the organization’s defense and missions; devotion of sufficient resources to armed forces to be able to meet the commitments of membership; security of sensitive information, and safeguards ensuring it; and compatibility of domestic legislation with NATO cooperation.

These general commitments were translated into contextually-specific language to give meaning and structure to the process by which new countries could aspire to membership.

In 2002 at their summit in Prague, NATO leaders extended the PfP by adding the option of an Individual Partnership Action Plan (IPAP) to help countries prepare to undertake a Membership Action Plan in the future. In 2004, the fifth and largest expansion of NATO membership since the alliance’s founding brought Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia into NATO. In 2009, President Obama attended the NATO summit in Strasbourg that formalized the admission of Albania and Croatia in the sixth and most recent addition to NATO membership.

Which brings us to NATO’s Chicago summit, where alliance leaders plan to discuss the conclusion of the deployment of NATO forces in Afghanistan, last year’s engagement in Libya, and prospects for future “out of area” operations. The expansion of NATO membership will be discussed in relation to the status of applicants with a signed Membership Action Plan (Bosnia Herzegovina, Macedonia and Montenegro) and those with current Individual Partnership Action Plans (Ukraine, Azerbaijan, Armenia, Kazakhstan, Moldova, Georgia, Belarus and Serbia).

The list of prospective members reveals an alliance looking eastward to Asia for new members. Past expansions of NATO membership consolidated the defense of Europe, but proposed future expansions will draw the alliance further into the defense of vulnerable countries of the former Soviet Union—such as Georgia, a non-NATO member that Russia invaded in 2008.

The United States’ traditional European NATO allies have promoted eastward expansion to secure the frontiers of the European Union. Europe’s integrated economy needs this security to reduce the risks of transnational organized crime, terrorism, and cyberattacks. NATO helps Europe to do more than defend against traditional military attacks; it provides a way to coordinate security and intelligence sharing across the region.

For Canada and the United States, the only current non-European NATO members (if one ignores Turkey for the moment) and charter members of the alliance, the eastward expansion of NATO membership extends our security guarantees farther and farther from our shores. Protecting Western Europe from Soviet nuclear attack during the Cold War was an act of friendship that also self-interestedly bolstered the security of both countries’ major trading partners. Mutual defense was mutually beneficial.

It is therefore surprising that North America’s NATO members have not proposed that Mexico be invited to participate in PfP with a view to eventual full NATO membership. Just as Europe’s NATO members have recognized that the extension of the alliance eastward is vital to the expansion of the European Union, Canada and the United States should have recognized by now that instability in Mexico is a threat to the economic prosperity and security of North American integration.

At a meeting of Canadian, U.S. and Mexican defense officials in Ottawa in March, the mutual vulnerability of the three countries to the threat of narco-trafficking and organized crime was acknowledged, along with a shared responsibility to address the problem. The United States and Canada have contributed to the Merida Initiative, a multilateral aid effort to help Mexico to improve its security.

The United States military’s Northern Command (NORTHCOM) has been coordinating with the Canadian military’s Canada Command in support of the Mexican government and its military. NORAD has been conducting satellite reconnaissance and aerial surveillance in support of anti-narcotics trafficking across North America for nearly a decade.

Since its founding in 1949, the NATO alliance has evolved to meet new threats. The security guarantees NATO members provide to one another have created zones of relative security in which free markets have flourished and raised the living standards in the devastation of postwar western Europe, the underdeveloped countries of southern Europe, the post Cold War republics of central and eastern Europe, and next perhaps in Eurasia.

Governments have, through NATO membership, relied on collective security and thus could shift public funds to education and social welfare rather than rearmament. U.S. complaints about free riding and inadequate burden-sharing within NATO notwithstanding, the benefits of NATO membership for economic well-being have led citizens of many countries to seek to join the alliance to lock in capitalism and good government reform.

Mexican security would gain tremendously from NATO membership, in part because the now-established process of negotiating an Individual Partnership Action Plan and subsequent Membership Action Plan would allow Mexico to proceed at its own pace along a proven path to security sector reform and modernization.

Canada could play a critical role as a longstanding ally of the United States used to managing the inherent asymmetry of that relationship while preserving Canadian sovereignty and dignity. Spain might undertake a major role in military exchanges given its common language and its NATO experience.

The United States would gain a framework for its sensitive but deepening security and intelligence cooperation with Mexico, easing bilateral anxieties through a multilateral institutionalization of the relationship between neighbours—it is worth recalling that U.S. troops withdrew from Mexico less than a century ago after a punitive campaign against bandits entangled the U.S. army in the Mexican revolution.

For Europe and the rest of NATO’s members, the expansion of the alliance in North America might reduce American resistance to a “European pillar” for regional security cooperation within NATO, as there could be a corresponding “North American pillar” developed in time.

President Obama and Prime Minister Harper should consider Mexico when they meet with other NATO leaders in Chicago. NATO with Mexico as a member could also confirm the alliance’s role as a guarantor of security and mutual cooperation against transnational security threats that contributes to the prosperity of the west, in Europe and North America equally. The future of this venerable pact would be assured by its utility in peacetime, not merely when threats emerge or explode onto the scene.