With a simple exchange of documents, Finland on Tuesday became NATO’s 31st member state, a strategic defeat for President Vladimir Putin of Russia, who was determined to block the alliance’s expansion but instead galvanized Finland to join amid Moscow’s devastating war in Ukraine.

Later in the day, Finland’s national flag was raised at NATO headquarters, a deeply symbolic moment and a stark display of the shifting global dynamics, as the West shores up its allegiances in response to Russia’s aggression toward its neighbor. With Finland’s membership assured, NATO doubled its borders with Russia and gained access to a strong military with a deep history of countering Russia.

NATO’s commitment to collective defense will now extend to a country that shares an 830-mile border with Russia and was twice invaded by its neighbor in the 20th century. If Finland is attacked, it can call on all members of the alliance for aid, a psychological and practical boost to Finns’ sense of security.