NATO Enlargement to Sweden and Finland

Sweden joined NATO on March 7, 2024, enlarging the alliance to 32 members (see Figure 1). Amid heightened security concerns about Russia after its February 2022 invasion of Ukraine, both Finland and Sweden applied to join NATO in May 2022 with the intention to accede to the alliance together. Finland joined on April 4, 2023, but Sweden's accession was delayed by issues raised by Turkey (Türkiye) and Hungary. The Biden Administration and majorities in the House and Senate expressed support for NATO enlargement to both Finland and Sweden, viewing the addition of these two mature democracies with capable militaries as enhancing Euro-Atlantic security and advancing U.S. interests.

Finland and Sweden Apply for NATO Membership

Finland's and Sweden's applications to NATO marked a historic shift for these traditionally militarily nonaligned countries that for decades sought to balance political, economic, and cultural ties to the West and relations with Russia. After the Cold War, Finland and Sweden joined the European Union (EU) and established close partnerships with NATO. Both Finland and Sweden have participated in numerous NATO operations over the years, and the two countries have advanced militaries that are interoperable with NATO. Public support for NATO membership—and the added security of NATO's "Article 5" mutual defense clause—skyrocketed in both countries following Russia's invasion of Ukraine; polls indicate support at 82% in Finland (as of November 2023) and 60% in Sweden (as of March 2024).

U.S. officials from the Departments of State and Defense concurred with Finnish and Swedish assertions that the two countries would strengthen NATO's defense posture, especially in the Baltic Sea region. Finland spends nearly 2.5% of its GDP on defense (above NATO's 2% spending guideline). Sweden intends to meet NATO's 2% defense spending goal in 2024.

Some analysts expressed concern about NATO's ability to defend Finland's 830-mile border with Russia (which doubles NATO's overall borders with Russia). Finnish officials contend that Finland has long defended its own borders and will continue to do so as a NATO member. Finland and Sweden also have strong traditions of societal resilience and experience dealing with Russian disinformation efforts, cyberattacks, and hybrid threats. Russia has largely sought to downplay concerns about Finnish and Swedish accession to NATO, but Russian President Vladimir Putin has warned NATO against deploying troops or expanding military infrastructure in either country. In the period before Finland's and Sweden's accession, the United States and NATO enhanced defense cooperation and conducted more joint training exercises with Finland and Sweden, in part to provide security reassurances to both countries.

The Path to NATO Accession

For a new member to be admitted to NATO, all allies must approve. Allied governments unanimously agreed to invite Finland and Sweden to join NATO at the alliance's June 2022 Madrid Summit, following a trilateral agreement concluded to address terrorism and other security concerns raised by Turkey and gain Turkey's support for the formal start of NATO's accession process. In July 2022, allied governments signed protocols to NATO's founding North Atlantic Treaty that had to be ratified by each ally (according to national procedures) to allow Finland and Sweden to join the alliance.

Twenty-eight of NATO's then-30 allies concluded the ratification process for both Finland and Sweden by late September 2022. Turkey, however, remained concerned about what it viewed as Swedish—and to a lesser extent Finnish—support for pro-Kurdish and other groups that Turkey regards as terrorist entities. In March 2023, Turkish President Recep Tayyip Erdogan announced his satisfaction with Finnish efforts to address Turkey's concerns; Turkey's parliament subsequently ratified Finland's accession but not Sweden's. Turkey continued to press Sweden to grant Turkish extradition requests and take a harder line against pro-Kurdish groups. Hungary's issues with Finland and Sweden were more opaque but largely centered on grievances over past EU and Swedish criticisms of Hungarian government policies perceived as eroding the rule of law. Hungary's parliament approved Finland's accession in March 2023 but delayed voting on Sweden's.

Swedish and NATO officials maintained that Sweden's accession was not a question of "whether" but "when." Sweden continued working to address Turkey's concerns, including by passing new anti-terrorism legislation. In July 2023, Turkish President Erdogan agreed to move forward with ratifying Sweden's accession. President Erdogan also publicly linked parliamentary approval of Sweden's accession to a possible U.S. sale of F-16 fighter jets to Turkey (although U.S. officials asserted that the two issues were separate). Turkey ultimately ratified Sweden's accession in late January 2024, and Hungary did so in late February 2024. Following Turkey's ratification, the Biden Administration formally notified Congress of the intended F-16 sale to Turkey. A new defense industrial agreement between Sweden and Hungary (including a deal to sell four Swedish-built Gripen fighter jets to Hungary) may have helped facilitate Hungary's approval of Sweden's accession.

Congressional Role and Interests in the Accession Process

In the United States, once NATO members sign accession protocols, the Administration sends them to the Senate for its advice and consent as part of the ratification process (see CRS Report 98-384, Senate Consideration of Treaties). President Biden submitted accession protocols for Finland and Sweden to the Senate on July 11, 2022. The Senate Foreign Relations Committee subsequently reported favorably on the resolution of ratification, and the full Senate approved it on August 3, 2022, by a vote of 95-1. President Biden signed the accession protocols for both Finland and Sweden on August 9, 2022, completing the U.S. ratification process. The House also passed a resolution of support for Finland and Sweden's NATO accession (H.Res. 1130) in July 2022 by a vote of 394-18.

Some Members of Congress expressed concerns about Turkey's and Hungary's delays in approving Finland and Sweden as NATO members. In February 2023, for example, a group of 29 Senators asserted that Turkey's delay "threatens the Alliance's unity" and noted they would not consider supporting the U.S. sale of F-16s to Turkey until Turkey ratified Finnish and Swedish accession. In February 2024, some Members also urged Hungary to swiftly ratify Sweden's accession.

Figure 1. NATO Members and Aspirants

Source: CRS Graphics. Map updated March 2024.