Sweden has formally joined NATO as the 32nd member of the transatlantic military alliance, ending decades of post-World War II neutrality. Sweden's membership comes as concerns about Russian aggression in Europe have spiked following Russia's 2022 invasion of Ukraine. Swedish Prime Minister Ulf Kristersson and U.S. Secretary of State Antony Blinken presided at a Washington ceremony Thursday when the "instrument of accession" for Sweden to join NATO was deposited at the State Department.