Nato's primary purpose was to block expansion in Europe by the Soviet Union - a group of communist republics which included Russia.
Members agree that if one of them is attacked, the others should help defend it.
Nato does not have an army of its own, but member countries can take collective military action in response to crises.
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Finland - which has a 1,340km (832 mile) land border with Russia - joined in April 2023. Sweden became a member in March 2024.
Both had applied to join in May 2022, shortly after Russia invaded Ukraine, having been neutral for decades.
Ukraine, Bosnia and Herzegovina, and Georgia also hope to join Nato.
Putin has managed to strengthen NATO by demonstrating that Russia is an immediate threat to the rest of Europe. Good going, Vladimir.