Sweden officially joined NATO on Thursday, becoming its 32nd member.

Why it matters: The nordic nation's accession to the alliance marks another major blow to Russian President Vladimir Putin, who has claimed that Moscow invaded Ukraine in hopes of blocking the alliance's eastward expansion.

• Swedish Prime Minister Ulf Kristersson handed U.S. Secretary of State Antony Blinken the final documentation in Washington, D.C., on Thursday to make the move official.
• "Thank you all Allies for welcoming us as the 32nd member," Kristersson said in a post on X. "We will strive for unity, solidarity and burden-sharing, and will fully adhere to the Washington Treaty values: freedom, democracy, individual liberty and the rule of law. Stronger together."

Flashback: Galvanized by Russia's unprovoked invasion of Ukraine, Sweden and Finland applied to join NATO in May 2022.

• Finland officially joined the alliance last April, which more than doubled NATO's borders with Russia.
• Their memberships notably marked the end of each country's long history of nonalignment.

What they're saying: "Swedes realized something very profound, that if Putin was trying to erase one neighbor from the map, then he might well not stop there," Blinken said after the documentation transfer.

• NATO's Secretary General Jens Stoltenberg marked the historic moment in a post on X.

It’s official – #Sweden is now the 32nd member of #NATO, taking its rightful place at our table. Sweden’s accession makes NATO stronger, Sweden safer, and the whole Alliance more secure. I look forward to raising their flag at NATO HQ on Monday.

— Jens Stoltenberg (@jensstoltenberg) March 7, 2024

Zoom in: The two Nordic states fortify NATO's northern flank and reduces the possibility of the Baltic states being isolated from other members if a military conflict were to erupt with Russia.

• Currently, the only land corridor between the Baltic states and the rest of Europe is the strategic territory known as the Suwałki Gap, which separates the Russian exclave of Kaliningrad from Belarus, which forms a union state with Moscow.
• With Finland's and Sweden's memberships, NATO now borders most of the the Baltic Sea, potentially allowing supplies and reinforcements to instead arrive to the Baltic states by sea if war broke out and the gap collapsed.

Catch up quick: Hungary ratified Sweden's accession in February after delaying its decision for more than 18 months.

• Hungarian Prime Minister Viktor Orbán, widely seen as the most pro-Russian leader in the European Union, ultimately endorsed the accession after Sweden agreed to send Hungary fighter jets.
• That was Sweden's last hurdle to membership, as joining requires unanimous approval from all NATO countries.

Go deeper: Yellen says House GOP delay on Ukraine aid is "a gift to Putin"

Editor's note: This story was updated with additional reaction and details.