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Why did Finland and Sweden seek NATO membership after Russia invaded Ukraine?

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Flag of NATO featuring a white compass rose emblem with four lines radiating from the center on a blue background. — North Atlantic Treaty Organization Flag of the North Atlantic Treaty Organization (NATO).

Following the Russian invasion of Ukraine in February 2022, both Finland and Sweden abandoned their long-standing traditions of neutrality to seek membership in the North Atlantic Treaty Organization (NATO).123

The decision was driven by the urgent need for enhanced security and collective defense in the face of Russian aggression, which was widely perceived as the greatest threat to European stability since World War II. By joining the alliance, these nations sought the protection afforded by NATO's collective deterrence, effectively transforming the Baltic Sea into a region of increased NATO presence. Finland, which shares an 830-mile border with Russia, officially became the 31st member of the alliance in April 2023, while Sweden followed as the 32nd member in March 2024.123