Finland and Sweden Invited to Join NATO: Significance and Process

by | Jun 30, 2022


On 29-30 June in Madrid, NATO held what its Secretary-General, Jens Stoltenberg, labeled a “historic and transformative” summit. Indeed, it was. And unsurprisingly so, for as the Secretary-General noted, NATO is facing the “most serious crisis [it] has faced since the Second World War.”

The Madrid Summit Declaration issued by the NATO Heads of State and Government left no doubt about the source of that crisis.

We condemn Russia’s war of aggression against Ukraine in the strongest possible terms. It gravely undermines international security and stability. It is a blatant violation of international law. Russia’s appalling cruelty has caused immense human suffering and massive displacements, disproportionately affecting women and children. Russia bears full responsibility for this humanitarian catastrophe. Russia must enable safe, unhindered, and sustained humanitarian access. Allies are working with relevant stakeholders in the international community to hold accountable all those responsible for war crimes, including conflict-related sexual violence. Russia has also intentionally exacerbated a food and energy crisis, affecting billions of people around the world, including through its military actions. Allies are working closely to support international efforts to enable exports of Ukrainian grain and to alleviate the global food crisis. We will continue to counter Russia’s lies and reject its irresponsible rhetoric. Russia must immediately stop this war and withdraw from Ukraine. Belarus must end its complicity in this war.

This situation led to NATO’s adoption of a new Strategic Concept at the summit. The concept is a dramatic change from that which it replaced. Adopted at the 2010 Lisbon Summit in the presence of then-Russian President Medvedev, the earlier document asserted,

NATO-Russia cooperation is of strategic importance as it contributes to creating a common space of peace, stability and security. NATO poses no threat to Russia. On the contrary: we want to see a true strategic partnership between NATO and Russia, and we will act accordingly, with the expectation of reciprocity from Russia.

By contrast, its 2022 counterpart observes,

The Euro-Atlantic area is not at peace. The Russian Federation has violated the norms and principles that contributed to a stable and predictable European security order. We cannot discount the possibility of an attack against Allies’ sovereignty and territorial integrity.

Any change in our relationship depends on the Russian Federation halting its aggressive behaviour and fully complying with international law.

It is an appropriately assertive strategy in which the Allies commit to “deter and defend forward with robust in-place, multi-domain, combat-ready forces, enhanced command and control arrangements, prepositioned ammunition and equipment and improved capacity and infrastructure to rapidly reinforce any Ally, including at short or no notice.”

A strategic shift was inevitable given Russia’s aggression against Ukraine. Other decisions and positions taken at the summit, ranging from a warning that China is “challeng[ing] our interests, security, and values and seek[ing] to undermine the rules-based international order to labeling climate change a “a defining challenge of our time with a profound impact on Allied security,” are also noteworthy. But in terms of altering the on-the-ground geostrategic equation in the Euro-Atlantic space, NATO’s membership invitation to formerly non-aligned Finland and Sweden is perhaps the most significant outcome of the Madrid Summit. This post surveys the operational significance of their joining NATO, the process by which they will become Allies, and the central obligation and right they will shoulder as such, collective defense.

Operational Significance

Expansion of the Alliance to include Finland and Sweden is operationally critical. The geographical significance of the two countries is difficult to overestimate. Finland shares an 830-mile border with Russia, including near the Kola peninsula, where Russian nuclear submarines and the Northern Fleet are based. Additionally, working with NATO member Estonia, Finland can control entry into and departure from the Gulf of Finland and, therefore, maritime access to St. Petersburg. NATO membership for the two countries would also mean that the Alliance surrounds the Baltic Sea except for Russia’s Kaliningrad enclave. Of particular geographical significance with regard to control of the Baltic Sea is the Swedish island of Gotland. And the location of the countries dramatically bolsters the defensive situation for NATO’s vulnerable Baltic members – Lithuania, Latvia, and Estonia.

Moreover, Finland’s and Sweden’s armed forces will contribute meaningfully to NATO’s fighting power. As of November 2021, Finland’s military includes over 19,000 active-duty troops, plus a reserve force of 238,000. The army fields 300+ tanks, and its Air Force has over 100 combat aircraft. Sweden’s armed forces approach 15,000 active-duty troops and 10,000 reserves. Its tank force is larger than Finland’s at just over 500, and it has approximately 100 combat aircraft and five submarines.

Both countries have long cooperated with NATO. They joined NATO’s Partnership for Peace in 1994 and have deployed personnel to NATO-led operations in the Balkans, Iraq, and Afghanistan. Around the time Russia invaded Ukraine in 2014, Finland and Sweden (together with Australia, Georgia, and Jordon ) became NATO Enhanced Opportunity Partners (EOP) in the Partnership Interoperability Initiative that was agreed to at the Wales Summit. As such, they benefit from tailor-made relationships with NATO designed to facilitate political consultations on security matters, enhanced access to interoperability programs, joint exercises with the Alliance, and a close association with NATO during crises and operations. In June 2020, Ukraine joined the group of EOPs.

Joining the Alliance

Article 10 of the 1949 North Atlantic Treaty (also known as the Washington Treaty) sets forth the legal basis for becoming a member of the Alliance. In relevant part, it provides, “The Parties may, by unanimous agreement, invite any other European State in a position to further the principles of this Treaty and to contribute to the security of the North Atlantic area to accede to this Treaty.” Since 1949 the Alliance has grown from 12 to 30 members with eight rounds of enlargement, the most recent being in 2020 when the Republic of North Macedonia joined. Enlargement is consistent with NATO’s “Open Door Policy,” which is the policy implementation of Article 10.

On 18 May 2022, Finland and Sweden sent letters of application to NATO. The letters were meant to quickly trigger an invitation from NATO to join the Alliance. However, Turkey expressed concerns about both countries’ perceived unwillingness to deal with Kurdish separatists and individuals accused of complicity in the 2016 coup attempt, as well as the informal arms embargo imposed on Turkey. Since an invitation requires unanimity, Turkey’s opposition could have blocked Finland and Sweden’s effort to join NATO at a time when membership is critical for both them and for the Alliance.

Turkish opposition was overcome the day before the summit opened by means of a formal trilateral memorandum between Turkey, Finland, and Sweden. In it, Finland and Sweden agreed not to support the Syrian Kurdish People’s Protection Units (YPG) and the associated Democratic Union Party (PYD), condemned terrorist attacks against Turkey, labeled the Kurdistan Worker’s Party (PKK) a terrorist group and committed themselves to help prevent its activities, confirmed that no arms embargoes targeting Turkey were in place, agreed to address Turkish concerns over the extradition of terror suspects, and committed to supporting Turkey’s involvement in European Union Common Security and Defence Policy activities, the framework for which the 2009 Lisbon Treaty set. For its part, Turkey agreed that it would fully support “Finland and Sweden against threats to their national security.” It also confirmed the principle of collective defense (discussed below), which is found in Article 5 of the North Atlantic Treaty and is an expression of the right of collective self-defense set forth in Article 51 of the UN Charter.

With Turkish concerns seemingly addressed, NATO formally extended an invitation to join the Alliance on the first day of the summit. The speed of the process from application to invitation was unprecedented. Usually, States expressing an interest in joining are invited to engage in an “intensified dialogue” with NATO about their aspirations and any reforms that the Alliance may require. In part, this is to ensure that aspirants must fulfill the criteria outlined in the 1995 Study on NATO enlargement. They include a democratic political system, a market economy, fair treatment of minority populations, commitment to peaceful resolution of disputes, ability to make meaningful contributions to NATO operations, and a commitment to democratic civil-military relations and structures.

Some countries participate in a Membership Action Plan (MAP) that will prepare them for membership, albeit not guarantee membership. Bosnia and Herzegovina joined the MAP process in 2010. Note that the Allies also agreed at the 2008 Bucharest Summit that Georgia and Ukraine would join NATO at some indefinite date in the future. Given the military wherewithal, political stability, and institutional maturity of Finland and Sweden, however, such steps were unnecessary.

Now that NATO has formally invited Finland and Sweden to join, they will begin accession talks. These discussions with officials at NATO headquarters are designed to ensure that the two countries can meet the political, legal, and military obligations set forth in the Washington Treaty and the aforementioned 1995 study. When reforms are necessary, accession talks result in a timetable for completion. In this case, NATO is unlikely to request significant reforms, and it is unimaginable that the accession talks could derail the process.

Finland and Sweden will then confirm their acceptance of the obligations and commitments of membership in letters from their Foreign Ministers to the NATO Secretary-General. At that point, NATO will prepare “accession protocols” to the North Atlantic Treaty that effectively amend the instrument, thereby permitting the two aspirants to become members of the Alliance. Each of the 30 NATO member States will have to ratify those protocols in accordance with their internal treaty ratification process. In the United States, for instance, the Senate must, by a 2/3 vote of senators present, give its advice and consent to the protocols. This will empower the President to ratify them (Article II, section 2). By contrast, in the United Kingdom, Parliament has no formal role in ratifying treaties, which is the government’s (executive branch) responsibility It is this step, which will likely take about a year, that poses the most significant risk, for a State, such as Turkey, may change its position.

Finally, each NATO member State will notify the United States, which is the depository of the North Atlantic treaty, of its acceptance of the protocols. The Secretary-General will then invite Finland and Sweden to accede to the treaty. Once they deposit their instrument of accession with the United States, Finland and Sweden will become members of the Alliance.

Collective Defense

As Allies, Finland and Sweden will shoulder numerous legal and policy commitments and obligations, such as contributing forces to NATO operations and providing various forms of direct and indirect funding, including a commitment to devoting at least 2% of their GDP to defense spending. But the core commitment, and NATO’s raison d’etre, lies in the collective defense obligation found in Article 5 of the North Atlantic Treaty.

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.

As is clear from the text, Article 51 of the UN Charter is the legal basis for the agreement among NATO’s member States to engage in collective defense. It provides, “Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security.” This treaty right reflects customary international law (International Court of Justice, Paramilitary Activities, ¶ 193).

The only time the North Atlantic Council (NAC, NATO’s principal decision-making body) has approved collective defense is in response to the al Qaeda attacks of September 11, 2001. The Alliance invoked Article 5 the following day. That invocation led to the deployment of NATO airborne warning and control system (AWACS) aircraft to monitor US airspace in Operation Eagle Assist and Operation Active Endeavor, an Eastern Mediterranean maritime counterterrorist operation. The fact that the NAC has only invoked Article 5 once should not belie its importance. Indeed, Finland and Sweden’s decision to dispense with over half a century of non-alignment is the direct result of the protection they believe NATO’s collective defense commitment will afford them.

A careful parsing of Article 5’s text reveals that the obligation to engage in collective defense is not absolute. Rather, Allies are only obligated to assist a fellow member State in collective self-defense once all 30 NATO nations agree that there has been an “armed attack” triggering the right of self-defense under Article 51 of the U.N. Charter. The unanimity requirement is a critical limiting factor, for a single Ally has the legal authority to block invocation of Article 5.

This has led to disagreement among scholars over whether Article 5 is truly binding (see, e.g., here, p. 214). My view is that “a State acting in good faith pursuant to the principle of pacta sunt servanda, as is required by the law of treaties, must not block invocation of Article 5 when an armed attack unambiguously occurs against a member of the Alliance.”

Of course, determining whether an armed attack has occurred can be problematic, especially in the cyber context. For instance, the 2022 Strategic Concept reiterates NATO’s view that a cyber attack could qualify as an armed attack for Article 51 purposes, but that view has not been universally embraced publicly. Other unsettled issues in the law of self-defense, such as whether the right extends to attacks by non-state actors and the scope of anticipatory self-defense, apply mutatis mutandis in the NATO collective self-defense context.

Certain issues, however, are unique to Article 5 collective defense. The first is the geographical reach of the article. Article 5 refers to an armed attack against a member of the Alliance in “Europe or North America.” Article 6 further develops this limitation.

For the purpose of Article 5, an armed attack on one or more of the Parties is deemed to include an armed attack:

      • on the territory of any of the Parties in Europe or North America, on the Algerian Departments of France [the NAC has since declared that the reference to Algerian territory is no longer applicable], on the territory of Turkey or on the Islands under the jurisdiction of any of the Parties in the North Atlantic area north of the Tropic of Cancer;
      • on the forces, vessels, or aircraft of any of the Parties, when in or over these territories or any other area in Europe in which occupation forces of any of the Parties were stationed on the date when the Treaty entered into force or the Mediterranean Sea or the North Atlantic area north of the Tropic of Cancer.

This geographical scope provision is all the more important in light of the concern expressed in the Madrid Summit Declaration about China. By it, an armed attack by China against U.S. forces in Asia, for instance, would not trigger Article 5. Of course, Finland, Sweden, or any other nation could come to the assistance of the United States in collective defense under Article 51 of the U.N. Charter, for the applicability of Article 5 is not a condition precedent to the exercise of collective defense by NATO states operating outside the NATO framework.

Notably, however, if China, North Korea, or another State (or non-State actor in my view) conducted an armed attack on U.S. territory, such as against U.S. forces stationed on Guam, Article 5 would apply. Moreover, an armed attack need not be conducted against a State’s armed forces on its own territory. As an example, a Russian attack on any NATO country’s forces that are deployed in Europe or North America, as with U.S. forces in Poland, would trigger the article. In such cases, Finland and Sweden would be obligated to assist them in collective defense.

This raises the material scope of the obligation. It is textually clear that Article 5 requires NATO States to consider an attack on one of them as an attack on all and provide assistance directly or in collaboration with other States. However, while the article imposes an obligation to assist, the decision of how that assistance will be provided is left to the individual member States, a point apparent from the “as it deems necessary” text. Thus, Finland and Sweden will not assume any obligation to engage in particular collective defense operations by having become members of NATO.

Moreover, Article 11 of the North Atlantic Treaty provides that its provisions are to be “carried out by the Parties in accordance with their respective constitutional processes.” This means that even if Finland, Sweden, or another NATO country has not blocked the invocation of Article 5 in the NAC, its commitment of forces would still be subject to limitations resident in domestic law processes and authorities.

Although Article 5 lies at the heart of the Alliance, NATO engages in military operations on several other bases, some of which Finland and Sweden have supported in the past and are even more likely to do so in the future. These include so-called “Non-Article 5 Crisis Response Operations.” The diversity of such operations is striking. They have ranged from Hurricane Katrina assistance to combat operations in Libya and Afghanistan.

The legal basis for NATO military operations varies. They are on their firmest footing when engaged in pursuant to a U.N. Security Council resolution under Chapter VII, as in Libya and Afghanistan. NATO is on thinner ice when mounting operations involving the use of force without Security Council imprimatur, as in NATO air operations against the Federal Republic of Yugoslavia in 1999 during Operation Allied Force. There is no obligation on the part of individual NATO members to participate in such operations. Finland and Sweden will be entitled to choose those they wish to join and how any support will be provided.

It should also be noted that Finland and Sweden may participate in “enhanced collective defense” measures under Article 4 of the North Atlantic Treaty. The article states, “The Parties 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.” Article 4 operations are initiated to deter aggression and, should it become necessary, facilitate Article 5 collective defense operations. Examples include NATO’s deployment of Patriot missiles in 1991 to Turkey during the first Gulf War and in 2012 during the Syrian conflict to provide counter-missile capabilities. Other examples include Operation Display Deterrence, which involved the deployment of AWACS aircraft, missile defenses, and chemical and biological defense equipment to Turkey as the situation in Iraq deteriorated, and the air policing missions in the Baltic region.


Hopefully, the accession process for Finland and Sweden will go smoothly, for the two countries bring significant military power and geographical advantage to an Alliance facing Russian aggression in its neighborhood. It could be accomplished as quickly as a year, although that will depend on how the ratification process goes in NATO’s 30 member States. And, as discussed, observers should be cautious about reading too much into the Article 5 obligation, although the limitations do not detract from its criticality in the current Euro-Atlantic security environment.


Michael N. Schmitt is the G. Norman Lieber Distinguished Scholar at the United States Military Academy at West Point. He is also Professor of Public International Law at the University of Reading; Professor Emeritus and Charles H. Stockton Distinguished Scholar-in-Residence at the United States Naval War College; and Strauss Center Distinguished Scholar and Visiting Professor of Law at the University of Texas.



Photo credit: NATO