GEOFF BENNETT: Today, Finland became the 31st member of the NATO alliance less than one year after it petitioned with Sweden to join.

Finland's accession doubles NATO's border with Russia, and it concludes an historic shift following 75 years of nonalignment.

Nick Schifrin explores the significance of expanding NATO.

NICK SCHIFRIN: For decades, Finland relied on a strong military to protect itself from Russia.

But Russia's war in Ukraine flipped the opinions of Finns and their leaders.

And, today, Finland's flag rose for the first time at NATO headquarters.

Finland's president and NATO secretary-general emphasized, an attack on Finland would now be considered an attack on all of NATO.

SAULI NIINISTO, President of Finland: The era of military nonalignment in our history has come to an end.

A new era begins.

JENS STOLTENBERG, NATO Secretary-General: NATO is a community united by our values, freedom, democracy and the rule of law.

We stand together.

We protect and defend each other.

All for one, and one for all.

NICK SCHIFRIN: To discuss this returned to Heather Conley, president of the German Marshall Fund of the United States, which focuses on improving transatlantic relations.

Heather Conley, thank you very much.

Welcome back to the "NewsHour."

How significant is this today?

And how does Finland militarily actually help contribute to NATO's security?

HEATHER CONLEY, Former State Department Official: Well, this was an incredibly historic day.

And it's a special day for NATO, because on this day 74 years ago, NATO was founded in Washington.

So it's a great birthday president to have the 31st member of NATO join Finland.

You're absolutely right.

Finland has an over-800 mile border with Russia.

But they have focused on their own territorial defense for decades.

They are a militarily capable ally.

And they -- job one for them is defending Finland and now, of course, part of NATO.

NICK SCHIFRIN: Putin, of course, for years has opposed NATO's expansion to Russia's borders.

How much of a strategic setback do you think this is for the Kremlin?

HEATHER CONLEY: It's massive.

And it's all because of Vladimir Putin's decision on February 24 of last year for his full-scale invasion in Ukraine.

This was -- Finland was a country that wanted the closest possible relationship with NATO, but not to become a NATO member.

And when that full-scale invasion began, they completely changed their opinion.

They did not want to stand alone.

They did stand alone in 1939, when they fought Soviet forces in the Winter War in 1939.

They were able to beat them back.

But they had to suffer what was called Finlandization, which was basically Soviet -- Soviet Union had to approve everything that Finland did.

And, as President Biden often says, now Finlandization is no longer the word.

It's NATO-ization.

So this is a massive step back.

You hear very muted words from the Kremlin, because it's such a setback.

They threatened Finland, they threatened Sweden with nuclear attack if they would join NATO.

And now it's a very muted -- muted comment, because, quite frankly, this is a significant strategic loss for them.

NICK SCHIFRIN: Among those comments from Russia is this.

The deputy foreign minister warned that Russia would -- quote -- "strengthen its military presence near Finland and would take unspecified further steps if more non-Finland NATO forces were actually deployed into Finland."

How likely is that?

HEATHER CONLEY: It's really not likely that you would see a meaningful deployment of NATO forces.

Again, Finland has a very robust territorial defense.

You're likely to see, again, more exercises.

We have to integrate fully NATO into defense -- NATO defense planning, but you're not going to see major infrastructure or major forces.

Finland can take care of its own territorial defense.

But, again, the Kremlin is incorrect.

Over the last 12 months, they have removed Russian forces from Finland's border.

These land forces had been used in Ukraine and have not come back.

So, in many ways, the Finnish government is seeing an easier time on the border, fewer Russian forces.

But they are prepared.

They know they could suffer cyberattacks.

They know they -- this will be tested, but they are very ready.

They have been ready since they sent their application in to become NATO members.

NICK SCHIFRIN: And those cyberattacks, of course, U.S. officials also worried about Russian cyberattacks in Ukraine and the West moving forward.

You mentioned Sweden before.

And Finland's president said specifically today that Finland's membership is not complete without Sweden, which is still being held up by both Turkey and Hungary.

Why are those two countries holding it up?

HEATHER CONLEY: Yes, so both have different reasons.

Turkey has been very concerned that Sweden has not been as robust and the fight against terrorism, very specific for Turkey, which would be the PKK.

Now, Sweden has... (CROSSTALK) NICK SCHIFRIN: The Kurdish militant party that is decided by the U.S. and the E.U.

to be a militant group.

HEATHER CONLEY: Exactly.

Now, the -- Sweden and Turkey have certainly been in bilateral conversations.

Sweden has changed its constitution.

It has been as forward-leaning as it can.

But the Turkish government would like the return of several Kurdish opposition figures.

And this would be something that the Swedish government will have a great difficulty to do.

So this is being held up until after Turkey holds its parliamentary elections on May the 14th.

And everyone's very hopeful that, after that election and before the NATO summit in Lithuania in July, that we can get past this.

Hungary is, in some ways, riding on Turkey's coattails and blocking this, seeing if they could get any benefit with Sweden to try to lessen the European Union's strong concerns about Hungary's lack of rule of law and diminishment of its democracy.

It will move when Turkey moves.

And Turkey will only move on Sweden until after their election, we hope.

We need to put a lot of pressure on the Turkish government.

They did commit that they would bring Sweden and Finland into NATO.

They're working through these bilateral issues.

And we hope they expedite this immediately after their election.

NICK SCHIFRIN: Heather Conley of the German Marshall Fund, thank you very much.