Finland became the 31st member of the North Atlantic Treaty Organization — NATO — on Tuesday. Welcoming the newest member state, U.S. Secretary of State Antony Blinken said after a ceremony in Brussels that both Finland and the NATO alliance were "stronger and safer" for it having joined.  

"Finland has a highly capable military and has been an active participant in NATO-led operations; it also shares our values and strong democratic institutions," said the top U.S. diplomat. 

"We are confident Finland's membership will strengthen our collective defense and enhance our ability to respond to security challenges in the Euro-Atlantic area," Blinken said in a statement. "Russia's further invasion into Ukraine last year precipitated the very thing President Putin wanted to avoid: a stronger, more unified, Transatlantic Alliance."  

Here's why the expansion of defensive alliance created to keep the U.S., Canada, and Europe safe in the wake of World War II matters today:

NATO was formed in 1949 by 12 countries, including the U.S., Canada and Western European nations. Its purpose was to ensure collective security against the Soviet Union. After the collapse of the USSR, more countries joined the alliance and it has more than doubled in size.

The stated mission of NATO is "to guarantee the freedom and security of its members through political and military means." At the heart of the treaty that established the alliance is Article 5, which says an attack on one NATO member will be considered by the allies as an attack on all. In the event of such an attack, it says members will take measures "to restore and maintain international peace and security."

Before he launched the full-scale invasion of Ukraine on February 24, 2022, Russian President Vladimir Putin had long complained of NATO's eastward expansion in Europe and sought to prevent additional countries from joining. Having framed the spread of the alliance's influence and military presence on the ground toward Russia's border as a threat, he used it as one of the pretexts for his attack on Ukraine.

The Nordic nations, including Finland, had shown little interest in becoming NATO members until Russia expanded its war in Ukraine. Though Finland had acted as a close NATO partner for many years, it was officially non-aligned.

The West's refusal to send troops into non-NATO member Ukraine to help it defend itself, however, laid bare the risks of non-alliance.

"If Ukraine had been part of NATO before the war, there would have been no war," Ukraine's President Volodymyr Zelenskyy said in May 2022.

Finland has its own history with Moscow, having been invaded by the Soviet Union in 1939, and since Russia's invasion of Ukraine, support for joining NATO within Finland has skyrocketed from about a quarter of the population, to 80%. The country applied for membership, along with neighboring Sweden, despite Russia's warnings against the move, in May 2022. 

Both countries' applications were held up by existing NATO member Turkey, however, with Ankara aggravated by support within the Nordic nations for Turkish opposition groups and Sweden allowing a protest that involved the burning of a Quran. Turkey recently ratified Finland's membership bid, but Sweden is still waiting for it to drop its resistance.

Finland is Russia's immediate neighbor to the west. The two countries share about 800 miles of land border,and Finland's membership in NATO will significantly bolster security on NATO's eastern flank. 

While Finland has said it does not need NATO troops stationed along its border with Russia "for now," its accession will give the alliance direct access to that 800-mile frontier, should it decide at any point to deploy additional forces for strategic or security purposes.

Just south of Finland are the Baltic states of Estonia, Latvia and Lithuania — all NATO members which also directly border Russia or its close ally Belarus. Those nations have long worried that Russia could seize Finnish islands to use as bases from which to stage attacks on their own territories. With Finland becoming a NATO member, they will be better protected. 

"President Putin went to war against Ukraine with the clear aim to get less NATO," the alliance's Secretary-General Jens Stoltenberg said Monday. "He's getting the exact opposite."

Russia has said it will bolster its own defenses in the west and northwest of its territory as a response to NATO's expansion.

CBS News correspondent Holly Williams contributed to this report.

In:
• War
• Finland
• Ukraine
• Russia
• Sweden
• Vladimir Putin
• NATO

Haley Ott

Haley Ott is an international reporter for CBS News based in London.

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