The latest: Finland has formally joined the North Atlantic Treaty Organization (NATO). Because the addition has more than doubled the land border between NATO and Russia, Vladimir Putin has threatened to retaliate, though Western leaders say their alliance is “more united than ever.”

Catching up: Article 5 of NATO states that an armed attack against one or more member states shall be considered an attack against all of NATO. In the case of an attack on Finland, all NATO allies, including the United States, are obligated to come to its defense and take necessary measures to restore and maintain security.

Why it matters: Finland’s border with Russia spans some 800 miles and long acted as a buffer zone, keeping a geographical distance between NATO and Russia. Additionally, one of Russia’s stated reasons for attacking Ukraine was an ever-expanding NATO border after the end of the Cold War. Some geopolitical experts warn that the lack of a “buffer zone” could increase the risk of nuclear war from misperception, faulty intelligence, or technological failures. Emma Ashford writes in the LA Times: