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.