Finland and Sweden are “like guesthouses for terror organizations,” according to Turkey’s President Recep Tayyip Erdoğan. On Wednesday, as Finland and Sweden formally applied to join the North Atlantic Treaty Organization, they faced Turkey’s opposition—with Erdoğan accusing them of harboring members of the Kurdistan Workers Party, a militant organization committed to its region’s separation from the Turkish state. The objection to Finland’s and Sweden’s membership in NATO was impossible to ignore—given that support from all 30 of the alliance’s existing member states is required to admit new countries—but it was also more a diplomatic hurdle than a barrier for the alliance, with the two Nordic democracies now almost certain to join. It’s a priority for them and for the alliance as a whole. But it’s also a major historical shift, after decades—in Sweden’s case, two centuries—of non-alignment. What’s happening?

Susan Colbourn is a NATO historian and the associate director of the Triangle Institute for Security Studies at Duke University’s Sanford School of Public Policy. According to Colbourn, Finland and Sweden’s actions represent a huge shift with a complex history—and the latest illustration of how Moscow’s invasion has led to acute unintended consequences for President Vladimir Putin and his country. Although Putin is likely to seize on the prospect of NATO expansion to continue his longstanding rhetorical assaults on the alliance, his practical options to respond are limited—especially given Russia’s weakened global position following the Ukraine invasion. As Colbourn sees it, the extension of NATO to Finland and Sweden is transformative—and uncertain, raising new questions as to how Putin will respond, as well as how the alliance will navigate a new era for European security.

Graham Vyse: Finland and Sweden went through the Cold War, and the decades since, without joining NATO. Why is that?

Susan Colbourn: When NATO was formed in 1949, in the early years of the Cold War, it was designed by the United States, Canada, and Western European leaders, from the perspective of how they saw the threat posed by the Soviet Union. At that time, it had been a decade since Finland had fought the Winter War against the Soviets—who invaded their country three months after the outbreak of World War II—and managed to avoid being entirely taken over. But the Soviet Union forced Finland to sign a treaty promising to adhere to a policy of military neutrality. As a result, throughout the Cold War, Western leaders and analysts would refer to “Finlandization” as a distinctive brand of neutrality—and a warning of what the Soviets might hope to achieve elsewhere in Europe.

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