(CNN) — Sweden officially joined NATO on Thursday after Russia's invasion of Ukraine prompted it to rethink its defense policy and abandon its long-held neutral stance.

Swedish Prime Minister Ulf Kristersson formally handed over membership documents to the US State Department in Washington, the final step in a months-long process to gain approval from all members and allow his country to become the 32nd member of the alliance.

The documents are kept in a vault in County administrationOr which acts as the depositary of NATO treaties.

Ulf Kristersson, Prime Minister of Sweden, on February 26, 2024. Photograph: Jonathan Nackstrand/AFP via Getty Images

“As we receive our instrument of accession, let me be the first to welcome Sweden as a party to the Washington Treaty and the 32nd member of NATO,” Secretary of State Antony Blinken said alongside Kristersson.

Kristersson thanked his allies for welcoming Sweden's accession to the bloc.

“We will strive for unity, solidarity, burden-sharing, and full commitment to the values ​​of the Washington Treaty: freedom, democracy, individual liberty and the rule of law. Together we are stronger,” he said in a statement.

Sweden and neighboring Finland, which shares a 1,300-kilometre border with Russia, sought to join the bloc after Moscow launched its all-out invasion of Ukraine.

Finland officially joined in April 2023, more than doubling NATO's border with Russia in a blow to President Vladimir Putin, who has sought to prevent any growth of the alliance.

But Sweden's attempt to join the bloc was blocked for months by Turkey and Hungary, which have friendlier relations with Moscow, and delayed ratifying Stockholm's accession.

The Turkish parliament voted in January to approve Sweden's nomination after months of tensions between the two countries, sparked mainly by a series of protests over the burning of the Holy Quran in Sweden that angered Turkish lawmakers.

This photo taken on February 27, 2024 shows an empty flagpole among the flags of member states at the Court of Honor at NATO headquarters, before the flag-raising ceremony for Sweden's accession to NATO, in Brussels on February 27, 2024. Credit: JOHN THYS / AFP via Getty Images

But Hungary has made Sweden wait longer, with Prime Minister Viktor Orban refusing to give the green light to its candidacy on several occasions. The Hungarian Parliament finally approved Sweden's accession last month.

NATO Secretary General Jens Stoltenberg said Sweden's accession “makes NATO stronger, Sweden safer, and the entire alliance safer.” “I look forward to raising your flag at NATO headquarters on Monday.”