Finland's flag was raised at Nato headquarters in Brussels on Tuesday in a solemn ceremony heralding a historic realignment of Europe's defences, triggered by Russia's invasion of Ukraine last year.
In becoming the alliance's 31st member, Finland moved ahead without its neighbour Sweden, which has been left on the waiting list due to Turkish objections.
“It is a great day for Finland,” said Finnish President Sauli Niinisto shortly before he watched his country's flag being raised.
The flag ceremony came minutes after Finnish Foreign Minister Pekka Haavisto gave his country's accession papers to US Secretary of State Antony Blinken, the keeper of Nato's founding treaty.
“With receipt of this document of accession, we can now declare that Finland is the 31st member of the North Atlantic Treaty,” Mr Blinken said.
The handover marked the moment Finland became an official member, an event that coincided with the 74th anniversary of the alliance.
US President Joe Biden said Nato was “stronger than ever”.
But Finland's accession to Nato, which roughly doubles the length of the border that the alliance shares with Russia, has angered Moscow.
“The Kremlin believes that this is the latest aggravation of the situation,” Kremlin spokesman Dmitry Peskov said.
“The expansion of Nato is an assault on our security and Russia's national interests.
“And this forces us to take countermeasures … in tactical and strategic terms.”
He did not provide further details.
Questioned by reporters about Russia's threats, Mr Niinisto said it was not up to the Kremlin to tell Finland what to do.
“Russians, in a way, tried to create a sphere around them and, well, we are not a sphere,” he said.
Nato Secretary General Jens Stoltenberg said Russian President Vladimir Putin had “wanted to slam Nato's door shut”.
“Today we show the world that he failed, that aggression and intimidation do not work,” he said.
“Finland now has the strongest friends and allies in the world.”
Sweden waits
Finland's first move as a Nato member was to endorse Sweden's accession bid and hand over its articles of ratification to Mr Stoltenberg.
Sweden had hoped to join Nato at the same time as Finland after the two countries filed a joint request in May after Russia invaded Ukraine.
Their bid was widely supported among Nato allies but Turkey made last-minute demands regarding Kurdish militant groups that Ankara views as a security threat.
Turkey's vetoing of Sweden's bid has frustrated its Nato allies, who believe that swift accession is necessary due to the war in Ukraine.
Many believe that Turkey is prioritising internal politics over the alliance's security.
Without explicitly mentioning Turkey, Danish Foreign Minister Lars Lokke Rasmussen said “you have to look at domestic politics in some countries to understand why they [Sweden] are not welcomed here today as full members”.
“I’m thrilled by the fact that we celebrate this Nato anniversary day by welcoming our Finnish friends,” he added. “I would have been even more thrilled if we could have welcomed Sweden as well.”
Swedish-Turkish negotiations are widely viewed as having reached a deadlock that only Turkey's presidential election in May can solve.
Asked by The National about whether the Turkish election would help his country's accession bid to Nato, Sweden's Foreign Minister Tobias Billstrom was hopeful.
“I hope so,” he said, speaking on the sidelines of a two-day meeting of Nato foreign ministers. “I think every country which has an election process is, of course, caught up with this, but after that, I think that we will see a progress.”
In a press conference, Mr Stoltenberg rejected rumours of a deadlock. He said talks were relaunched recently after he met Turkish President Recep Tayyip Erdogan last month.
“Sweden is not left alone. It's not like allies don't care about Sweden,” he told reporters.
“It is inconceivable that there [would be] any military attack or threat against Sweden without Nato reacting, and that is even more so with Finland inside [Nato].”
Most Nato members hope that Sweden will be able to join the alliance before its next summit of heads of governments in Lithuania in July.
The next plenary session of the Turkish parliament — during which MPs have the possibility to approve Sweden's accession bid — is scheduled for June.
Court ruling
In a separate development on Tuesday, Sweden lifted a ban on Quran-burning protests that had further angered Turkey.
Far-right politician Rasmus Paludan caused international condemnation when he set fire to the holy book in January.
But judges ruled that fears of violent reprisals against Sweden did not justify a ban on further actions outside the Turkish and Iraqi embassies in Stockholm.
The ruling came as five people with alleged links to ISIS were arrested over a suspected terrorist plot in Sweden amid “calls for attacks” after the Quran-burning incident.
“The security police often need to intervene early to ward off a threat. We cannot wait for a crime to be completed before we act,” said Susanna Trehorning, deputy counter-terrorism chief at Sweden's security police.
Speaking in Brussels, Mr Billstrom told reporters that his country valued freedom of expression but saw Quran-burning as inappropriate.
“I also want to underline that in the memorandum that we signed with Turkey, there is nothing mentioned about religion, nothing about those kinds of issues. We intend to do what we have committed to under the memorandum. But nothing more, nothing less,” he said.
Turkey did not immediately react to the Swedish court decision.
Mr Billstrom pointed to the fact that the decision was “not final”.
It “can be appealed to a higher court and ultimately to the supreme administrative court in Sweden”, he said, answering a question from The National.
Mr Billstrom added that the arrest of the five suspects was outside of his government’s responsibility, but he highlighted that Sweden had tightened its counter-terrorism legislation at Turkey’s request.
“We have stated clearly that we are to take action against terrorist groups working on our territory,” said Mr Billstrom. “Whether this is applicable in this case or not, I’m not in a position to go into.”
Swedish police had made no link between Tuesday’s arrests and Kurdish militant groups.
