Nato's border with Russia doubles as Finland joins
Finland has become the 31st member of the Nato security alliance, doubling the length of member states' borders with Russia.
The Finnish foreign minister handed the accession document to the US secretary of state who declared Finland a member.
Then in bright sunshine in front of Nato's gleaming new headquarters, Finland's white-and-blue flag joined a circle of 30 other flags.
Finland's accession is a setback for Russia's Vladimir Putin.
He had repeatedly complained of Nato's expansion before his full-scale invasion of Ukraine.
US Secretary of State Antony Blinken said by attacking his neighbour, the Russian leader had triggered exactly what he had sought to prevent.
Kremlin spokesman Dmitry Peskov warned that Russia would be "watching closely" what happens in Finland, describing Nato's enlargement as a "violation of our security and our national interests".
A military band played Finland's national anthem followed by the Nato hymn. Beyond the perimeter fence a small group of protesters waving Ukrainian flags chanted "Ukraine in Nato", a reminder of why non-aligned Finland had asked to join along with Sweden in May 2022.
Finland shares a 1,340-km (832-mile) eastern frontier with Russia and after the war in Ukraine began Helsinki chose the protection of Nato's Article Five, which says an attack on one member is an attack on all.
In effect, it means if Finland were invaded or attacked, all Nato members - including the US - would come to its aid.
Russia's invasion prompted a surge in Finnish public opinion towards joining Nato to 80% in favour.
"It is a great day for Finland," said Finnish President Sauli Niinisto, proclaiming a new era for his country. Finland would be a reliable ally and its membership would not be a threat to anyone, he said. "Security and stability are those elements which we feel very strongly; if people can live in secure stable circumstances that's the basic element of happy life."
"This will make Finland safer and Nato stronger," said Secretary General Jens Stoltenberg earlier, describing it as a proud day for him and the alliance.
"President Putin had a declared goal of the invasion of Ukraine to get less Nato along its borders and no more membership in Europe, he's getting exactly the opposite."
US Secretary of State Antony Blinken said he was "tempted to say this is maybe the one thing we can thank Mr Putin for, because he once again here has precipitated something he claims to want to prevent by Russia's aggression".
Finland brings with it a well-equipped and trained, active armed force of about 30,000 with a wartime strength of 280,000.
It also provides a challenge for Nato to help keep its long border with Russia secure, but it is already being included in Nato's latest defence plans to keep the alliance secure.