As the blue-and-white Finnish flag rose outside NATO headquarters on Tuesday, a new 1,340-kilometre line was being drawn in the sand.
Finland's membership into the world's largest military alliance has doubled the length of member states' borders with Russia.
It is a searing blow to Russian President Vladimir Putin, who has long complained about NATO's expansion toward Russia and partly used that as a justification for his country's war with Ukraine.
Already, Helsinki's strategic shift has drawn an angry warning of "countermeasures" from the Kremlin.
"What we see is that President Putin went to war against Ukraine with a declared aim to get less NATO," NATO Secretary-General Jens Stoltenberg said.
"He's getting the exact opposite."
The alliance's 31st member takes the NATO frontier to 2,500 kilometres.
So, what does the major expansion mean for Finland and what can we expect from NATO — and Russia — next?
A wall and a powerful military
Invaded by its giant neighbour, the Soviet Union, in 1939, Finland stayed out of NATO throughout the Cold War.
Now its membership brings a potent military into the alliance with a wartime strength of 280,000 and one of Europe's largest artillery arsenals.
It has also helped to complete NATO's geographical puzzle by filling in a major gap in the strategically important Baltic Sea region in Europe's north.
Professor Matthew Sussex, senior fellow at ANU's Strategic and Defence Studies Centre, said it was a "big boost" to NATO.
As the border runs more or less in a straight line with no rivers or lakes running through, Professor Sussex said Finland is likely to further militarise the area.
There are also suggestions that it will construct a wall between itself and Russia, he added.
"Likely with barbed wire and observation posts — that sort of thing."
Finland already has security such as dog patrols in areas where there isn't a huge population to stop people trying to illegally cross the border from Russia.
"Now it seems that that would be stepped up quite considerably," Professor Sussex told the ABC.
"That border will be the kind of fortified one."
No NATO troops on Finnish soil
Sharing an eastern border with Russia, the Nordic country applied to join the alliance soon after Mr Putin's full-scale invasion of Ukraine.
"Until now, we have defended our country alone," Finnish Defence Minister Antti Kaikkonen told public broadcaster YLE on arrival at the accession ceremony in Brussels.
"From now on, we can rely on getting outside help should things get tough. And of course, we are ready to help should someone be in trouble."
The ceremony in Brussels falls on the 74th anniversary of the signing of NATO's founding Washington Treaty on April 4, 1949.
Senior NATO military commander Admiral Rob Bauer told AFP that Finland had so far not requested its new allies station troops on its soil.
But like all NATO members, Finland will benefit from the collective security guarantee that an attack on one is considered an attack on all.
Under the alliance's Article Five, the collective defence pledge that an attack on one member "shall be considered an attack against them all".
"I don't think there are too many circumstances we need to send large sums of NATO troops to Finland simply because the Finnish military is extremely strong," Professor Sussex said.
Russian response 'fairly muted'
Though the alliance says it poses no threat to Moscow, the Russian Foreign Ministry warned that "it will be forced to take military-technical and other retaliatory measures" to a move that marks "a fundamental change in the situation in Northern Europe, which had previously been one of the most stable regions in the world".
Kremlin spokesman Dmitry Peskov also pointed out that Russia has no territorial disputes with Finland.
Russia said on Monday it would strengthen its military capacity in its western and north-western regions in response to Finland joining NATO.
NATO officials say the war in Ukraine has sapped Moscow's forces, but the alliance is monitoring how Russia responds to gauge its future steps.
Despite some bolshy rhetoric from Moscow, Professor Sussex said the response to Finland and Sweden's NATO bid was "fairly muted".
Sweden had submitted a joint application to join NATO with Finland in May 2022.
"Probably because they [Russia] realised that the invasion of Ukraine has actually brought about the expansion of NATO," Professor Sussex said.
In warnings to the West against arming Ukraine, Russian officials increasingly play up the risks of nuclear weapons being used in the war.
Last month they said they would station tactical nuclear weapons in neighbouring Belarus.
Is Sweden next to join NATO?
Finland's arrival nevertheless remains a bittersweet moment for the alliance as the hope had been for Sweden to come on board at the same time.
Helsinki's first act as a new member was to back Stockholm's bid.
Türkiye and Hungary remain the holdouts after belatedly agreeing to wave through Helsinki's bid.
Sweden has upset Hungary's leader Viktor Orban — one of Mr Putin's closest allies in Europe — by expressing alarm over the rule of law in Hungary.
It has also angered Türkiye by refusing to extradite dozens of suspects that President Recep Tayyip Erdogan links to a failed 2016 coup attempt and the decades-long Kurdish independence struggle.
Analysts do not expect Türkiye to ratify Sweden's application until after Turkey's May 14 election at the earliest.
Even then, it is unclear what would trigger a change of heart by Mr Erdogan.
The United States and other NATO members led the calls for Sweden to join as soon as Finland's flag was fluttering in the cold Brussels breeze.
German Chancellor Olaf Scholz said he believed both countries would take part in a NATO summit in Vilnius this summer as new members.
Ukraine is also pushing for eventual NATO membership, but Western diplomats say that remains a distant prospect.
ABC/Wires
