JUNE 29: Russia strongly opposes the two states joining and has used the expansion of the West's defensive military alliance as a pretext for its war in Ukraine.
Both countries have held neutral status for years, but since Russia's invasion of Ukraine support for Nato membership has risen dramatically.
Why join now?
Vladimir Putin's actions have shattered a long-standing sense of stability in northern Europe, leaving Sweden and Finland feeling vulnerable.
Finnish ex-Prime Minister Alexander Stubb said joining the alliance was a "done deal" for his country as soon as Russian troops invaded Ukraine on 24 February.
For many Finns, events in Ukraine bring a haunting sense of familiarity. The Soviets invaded Finland in late 1939. For more than three months the Finnish army put up fierce resistance, despite being heavily outnumbered.
They avoided occupation, but ended up losing 10% of their territory.
Watching the war in Ukraine unfold was like reliving this history, said Iro Sarkka, a political scientist at the University of Helsinki. Finns were looking at their 1,340km (830 mile) border with Russia, she said, and thinking: "Could this happen to us?"
Sweden has also felt endangered in recent years, with several reported airspace violations by Russian military aircraft.
In 2014, Swedes were transfixed by reports that a Russian submarine was lurking in the shallow waters of the Stockholm archipelago.
Two years later Sweden's army returned to the small but strategically important Baltic Sea island of Gotland, after abandoning it for two decades.
What will change?
In some ways, not much. Sweden and Finland became official partners of Nato in 1994 and have since become major contributors to the alliance. They have taken part in several Nato missions since the end of the Cold War.
The two countries will for the first time have security guarantees from nuclear states under Nato's Article 5, which views an attack on one member state as an attack on all.
Historian Henrik Meinander said Finns were mentally prepared for membership, following a succession of small steps towards Nato since the fall of the Soviet Union.
In 1992, Helsinki bought 64 US combat planes. Three years later, it joined the European Union, alongside Sweden, and every Finnish government since then has reviewed the so-called Nato option. The army, which serves a population of 5.5 million, has a wartime strength of 280,000 soldiers, and 900,000 reservists in total.
Sweden took a different path in the 1990s, reducing the size of its military and changing priorities from territorial defence to peacekeeping missions around the world. But that all changed in 2014, when Russia seized and annexed Crimea from Ukraine. Conscription returned and defence spending was boosted. In 2018, every household received army pamphlets titled "if crisis or war comes" - the first time they were sent out since 1991.
Finland has already reached Nato's agreed defence spending target of 2% of GDP, and Sweden has drawn up plans to do so.
What are the risks?
Russian President Vladimir Putin believes Nato expansion is a direct threat to his country's security, so Sweden and Finland joining the alliance will be perceived as a provocation.
Russia's foreign ministry says both countries have been warned of the "consequences" of such a move. Former President Dmitry Medvedev, a close ally of the Russian leader, has warned that Nato accession may prompt Moscow to deploy nuclear weapons in Kaliningrad, the Russian exclave between Poland and Lithuania.
While not dismissing these threats, Alexander Stubb suggested a more realistic risk was of Russian cyber attacks, disinformation campaigns and occasional airspace violations.
Will Nato make Sweden and Finland safer?
There is a significant minority, at least in Sweden, who believe it will not.
Deborah Solomon, from the Swedish Peace and Arbitration Society, argued that Nato's nuclear deterrence increased tensions and risked an arms race with Russia. This complicated peace efforts, she said, and made Sweden a less safe place.
Another fear is that in joining the alliance, Sweden would lose its leading role in global nuclear disarmament efforts. Many of Sweden's Nato-sceptics look back to the period between the 1960s and '80s, when Sweden used its neutrality to position itself as an international mediator.
Joining Nato would be abandoning that dream, Ms Solomon said.
Finland's neutrality was very different. It came about as a condition of peace imposed by the Soviet Union in a 1948 "friendship agreement". It was seen as a pragmatic way of surviving and maintaining the country's independence.
If Sweden's neutrality was a matter of identity and ideology, in Finland it was a question of existence, said Henrik Meinander. Part of the reason Sweden could even afford to have a debate about Nato membership was because it used Finland and the Baltics as a "buffer zone", he said.
Finland abandoned its neutrality after the Soviet Union collapsed. It looked to the West and sought to free itself from the Soviet sphere of influence.
Iro Sarkka suggested joining Nato was seen as too big a step for Finland to take in the early 1990s, having just emerged from neutrality.
But times and perceptions of risk have changed. Now, most Finns say they are ready.
What obstacles did they face to joining?
For weeks, Sweden and Finland's applications were held up by Turkey. Any Nato enlargement must be approved by all 30 members.
The Turkish government claimed the Nordic nations were supporting what it calls terrorist organisations, including Kurdish separatists and the Gulen movement, which Turkey blames for an attempted coup in 2016.
Kurds make up 15-20% of Turkey's population, and have been persecuted by Turkish authorities for generations.
In exchange for its support, Turkey said it wanted Sweden and Finland to stop providing political, financial and "arms support" to the groups.
It also wanted them to resume selling weapons to Turkey and hand over individuals with alleged terror links.
After hours of talks at the Madrid Nato summit in late June, foreign ministers from Sweden, Finland and Turkey signed a joint security pact addressing Turkey's concerns.
Nato chief Jens Stoltenberg said Sweden had agreed to step up its work on Turkish extradition requests of suspected militants. The two Nordic nations also said they would lift their restrictions on selling weapons to Turkey, he added.
In exchange, Turkey will lift its veto on the Nordic nations joining the alliance.
With inputs from BBC
MAY 20: Sweden and Finland have the "full, total and complete backing" of the US in their decision to apply for Nato membership, President Joe Biden says.
Both countries submitted their applications to be part of the Western defence alliance this week, marking a major shift in European geopolitics.
To join the alliance, the two nations need the support of all 30 Nato member states.
But the move by the Nordic nations has been opposed by Turkey.
Speaking alongside Swedish Prime Minister Magdalena Andersson and Finnish Prime Minister Sauli Niinisto at the White House on Thursday, Mr Biden called Sweden and Finland's applications "a watershed moment in European security".
"New members joining Nato is not a threat to any nation," he said. The president added that having two new members in the "high north" would "enhance the security of our allies and deepen our security co-operation across the board".
Russia has repeatedly said it sees Nato as a threat and has warned of "consequences" if the block proceeds with its expansion plans.
Turkey has accused both Sweden and Finland of hosting suspected militants from the Kurdistan Workers' Party (PKK), a group it views as a terrorist organisation.
However, both Nato Secretary-General Jens Stoltenberg and British Defence Minister Ben Wallace have expressed confidence that these concerns will eventually be addressed.
Mr Biden's comments came as the US Senate voted to approve a new $40bn (£32bn) bill to provide military and humanitarian aid to Ukraine. It is the biggest emergency aid package so far for Ukraine.
The bill - which was passed by the House of Representatives with broad bipartisan support on 10 May - was expected to be passed earlier this week, but was blocked by Kentucky Republican Rand Paul over a dispute about spending oversight.
But the Republican's Senate leader Mitch McConnell dismissed these concerns and told reporters that Congress had a "moral responsibility" to support "a sovereign democracy's self-defence".
"Anyone concerned about the cost of supporting a Ukrainian victory should consider the much larger cost should Ukraine lose," Mr McConnell said.
Last week, Defence Secretary Lloyd Austin and Secretary of State Antony Blinken urged Congress to approve the package and warned that the US military only had enough funds to send weapons to Kyiv until 19 May.
Ukraine's President Volodymyr Zelensky praised the aid package as "a significant US contribution to the restoration of peace and security in Ukraine, Europe and the world".
The package brings the total US aid delivered to Ukraine to more than $50bn, including $6bn for security assistance such as training, equipment, weapons and support.
Another $8.7bn will be allocated to replenish stocks of US equipment already sent to Ukraine.
MAY 15: He told Sauli Niinistö there was no threat to Finland's security.
The exchange came during a phone call made by the Finnish president, ahead of a formal request which Finland is expected to announce very soon.
Sweden has also indicated its intention to join the Western alliance, following Russia's invasion of Ukraine.
Finland shares a 1,300-km (810-mile) border with Russia. Until now, it has stayed out of Nato to avoid antagonising its eastern neighbour.
Mr Putin did not make a specific threat of retaliation over Finland's move, but the Russian foreign ministry has indicated there will be retaliation.
Russia's decision to suspended electricity supplies to Finland is being seen as an early sign, however.
In its statement, Russian energy supplier RAO Nordic mentioned problems with payments.
Finland national grid executive Reima Paivinen told the BBC the Russian suspension had not caused any problems.
He said Russian imports accounted for around 10% of national supply, adding that they could be replaced from alternative sources.
Following the phone call between Mr Niinistö and Mr Putin on Saturday, the Kremlin said the Russian leader had stressed the "end of the traditional policy of military neutrality would be a mistake since there is no threat to Finland's security".
It added: "Such a change in the country's political orientation can have a negative impact on Russian-Finnish relations developed over years in a spirit of good neighbourliness and co-operation between partners."
Mr Niinistö said he had told Mr Putin how recent moves by Russia, along with the invasion of Ukraine, "have altered the security environment of Finland".
"The conversation was direct and straight-forward and it was conducted without aggravations. Avoiding tensions was considered important," he said.
A possible stumbling block to Swedish and Finnish membership of Nato is Turkey, after President Recep Tayyip Erdogan accused both countries of harbouring "terrorist organisations" and said he did not support their applications.
It was seen as a reference to the Kurdistan Workers' Party (PKK), which Turkey regards as a terrorist organisation. For decades the PKK has fought with Turkey for a Kurdish homeland.
Turkey also objects to a Syrian Kurdish militia called the People's Protection Units (YPG) ,which they say is an extension of the PKK - even though they have been allied with the US in the fight against so-called Islamic State.
Both Sweden and Finland have Kurdish communities, and in Sweden's case some parliamentarians have Kurdish origins. President Erdogan did not provide any proof that these communities had links with the PKK.
Foreign ministers from Sweden, Turkey and Finland are meeting to try to iron out their differences.
Ahead of the talks, Turkish Foreign Minister Mevlut Cavusoglu echoed the comments of his president but signalled a willingness to talk.
"A big majority of the Turkish people are against the membership of those countries who are supporting PKK terrorist organization ... but these are the issues that we need to talk of course with our NATO allies as well as these countries," AFP quoted him as saying.
Nato, a Western military alliance, was founded in part to ward off threat from the Soviet Union in 1949.
President Putin has mentioned Ukraine's intention to join the alliance as one of the reasons for the invasion of its neighbour.
JAN 25: Russia wants the West to promise that Ukraine will not join its Nato defensive alliance, and although the two sides are negotiating, that is not going to happen. What happens next could jeopardise Europe's entire security structure.
Why is Russia threatening Ukraine?
Russia denies it's planning any invasion, but it has seized Ukrainian territory before and it has an estimated 100,000 troops deployed near its borders.
Russia has long resisted Ukraine's move towards European institutions, and Nato in particular.
Ukraine shares borders with both the EU and Russia, but as a former Soviet republic it has deep social and cultural ties with Russia, and Russian is widely spoken there.
When Ukrainians deposed their pro-Russian president in early 2014, Russia annexed Ukraine's southern Crimean peninsula and backed separatists who captured large swathes of eastern Ukraine. The rebels have fought the Ukrainian military ever since in a conflict that has claimed more than 14,000 lives.
How big is the risk of invasion?
Russia says it has no plans to attack Ukraine: and armed forces chief Valery Gerasimov even denounced reports of an impending invasion as a lie.
But tensions are high and President Vladimir Putin has threatened "appropriate retaliatory military-technical measures" if what he calls the West's aggressive approach continues.
Nato's secretary general warns the risk of conflict is real and President Biden says his guess is that Russia will move in. The US says it knows of Russian plans to boost its forces near Ukraine "on very short notice".
The US says Russia has offered no explanation for the troops posted close to Ukraine - and Russian troops and tanks have headed to Belarus for exercises.
Russia's deputy foreign minister compared the current situation to the 1962 Cuban missile crisis, when the US and Soviet Union came close to nuclear conflict.
Western intelligence suggests a Russian incursion or invasion could happen some time in early 2022.
What does Russia want from Nato?
Russia has spoken of a "moment of truth" in recasting its relationship with Nato: "For us it's absolutely mandatory to ensure Ukraine never, ever becomes a member of Nato," said Deputy Foreign Minister Sergei Ryabkov.
Moscow accuses Nato countries of "pumping" Ukraine with weapons and the US of stoking tensions. President Putin has complained Russia has "nowhere further to retreat to - do they think we'll just sit idly by?"
In reality Russia wants Nato to return to its pre-1997 borders.
It demands no more eastward expansion and an end to Nato military activity in Eastern Europe. That would mean combat units being pulled out of Poland and the Baltic republics of Estonia, Latvia and Lithuania, and no missiles deployed in countries such as Poland and Romania.
Russia has also proposed a treaty with the US barring nuclear weapons from being deployed beyond their national territories.
What does Russia want with Ukraine?
Russia seized Crimea in 2014 arguing it had a historic claim to it. Ukraine was part of the Soviet Union, which collapsed in December 1991 and Mr Putin said it was the "disintegration of historical Russia".
A clue to President Putin's thinking on Ukraine came in a lengthy piece last year when he called Russians and Ukrainians "one nation". He labelled Ukraine's current leaders as running an "anti-Russian project".
Russia has also become frustrated that a 2015 Minsk peace deal for eastern Ukraine is far from being fulfilled.
There are still no arrangements for independently monitored elections in the separatist regions. Russia denies accusations that it is part of the lingering conflict.
Can Russian action be stopped?
President Vladimir Putin has spoken several times to Mr Biden and high-level talks continue, but Russian officials have warned that Western rejection of their key demands are leading to a "dead end".
The question is how far Russia will go. President Biden has warned that a full-scale invasion would be a disaster for Russia. But if it was a minor incursion, he said controversially that the West would "end up having to fight about what to do".
The White House has stressed any move across the border constitutes a renewed invasion - but points out Russia has other weapons, including cyber-attacks and paramilitary tactics.
The Pentagon has accused Russia of preparing a so-called false-flag operation, with operatives ready to carry out acts of sabotage against Russian-backed rebels, to provide a pretext for invasion. Russia has denied it.
Russia has also handed out 500,000 passports in rebel-run areas, so if it does not get what it wants then it could justify any action as protecting its own citizens.
However, if Russia's only aim is to force Nato away from its backyard, there is no sign of it succeeding.
Nato's 30 members have turned down flat any attempt to tie their hands for the future. "We will not allow anyone to slam closed Nato's open-door policy," said US Deputy Secretary of State Wendy Sherman.
Ukraine is looking for a clear timeline to join and Nato says Russia has "no veto, no right to interfere in that process".
And non-Nato members Sweden and Finland have also rejected Russia's attempt to stop them beefing up their ties with the alliance. "We will not let go of our room for manoeuvre," said Finland's prime minister.
How far will the West go for Ukraine?
The US has made clear it has no plans to send combat troops, while being committed to helping Ukraine defend its "sovereign territory".
The main tools in the West's armoury appears to be sanctions and military aid in the form of advisers and weapons.
President Biden has threatened Russia's leader with measures "like none he's ever seen" if Ukraine is attacked. So what would they involve?
The ultimate economic hit would be to disconnect Russia's banking system from the international Swift payment system. That has always been seen very much as a last resort, but Latvia has said it would send a strong message to Moscow.
Another key threat is to prevent the opening of Russia's Nord Stream 2 gas pipeline in Germany, and approval for that is currently being decided by Germany's energy regulator.
There could also be measures targeting Russia's RDIF sovereign wealth fund or restrictions on banks converting roubles into foreign currency.
Is the West united in its response?
Washington has said it is committed to "working in lockstep" with its allies, but there are divisions between the US and Europe.
European leaders are adamant that Russia cannot just decide on the future with the US. France has even proposed that Europeans work together with Nato and then conduct their own dialogue with Russia.
BBC
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