Everything changed when Russia invaded Ukraine. Deterrence was turned on its head. Instead of preventing President Vladimir Putin from open aggression, the threat of nuclear war shielded him from intervention.
Now Europe is worried the ageing autocrat aims to recreate the long-lost empire of Tsar Peter the Great.
It's a raw nerve Ukraine's President Volodymyr Zelensky sought to trigger as Europe's leaders assembled to reset the future of the 30-member Nato alliance.
"This is Russia's real goal," he warned on Thursday while appealing for aid to stop the Russian army in its tracks. "There will be either urgent help for Ukraine – enough to win – or Russia's postponed war with you."
And that prospect, he intoned, could come sooner rather than later.
"Next year could be a worse situation – not only Ukraine, but also several other states. Possibly members of the alliance. The question is — who is next?"
It's a very real fear.
Sweden and Finland – which maintained their neutrality for the whole Cold War between the West and the Soviet Union – this week had their applications to join the Nato alliance accepted.
They believe they have urgent cause to join.
And Putin's propagandists have repeatedly stated his objectives go beyond Ukraine.
His apparatchiks have been threatening to invade Nato members Poland and Lithuania. Norway is another target of Moscow's ire. And the United Kingdom and the United States have been menaced with the spectre of a nuclear strike.
There's one catch.
Any attack on any one member of the Nato alliance triggers Article V: Every member state must consider it an assault on itself.
War warning
"The Russian army does not want to stop in Donbas or somewhere in the south of Ukraine," Zelenskyy stated. "It wants to absorb city after city. All of us. And then all in Europe whom the Russian leadership considers its property – not independent states. This is Russia's real goal."
The desperate Ukrainian leader has every motive to talk up the threat.
But his words are falling on fertile ground.
And the Kremlin appears to be doing all it can to validate his concerns.
This week, Deputy Foreign Minister Sergei Ryabkov declared the decision of Finland and Sweden to join Nato to be a "strictly destabilising factor".
At the same time, foreign ministry officials were threatening "retaliation" against Norway for imposing sanctions on supplies transported over its territory to a Russian mining outpost. And Lithuania was being accused of "an act of war" after imposing a "blockade" against Russia's Baltic Sea outpost of Kaliningrad.
Russia is free under international sanctions to resupply both locations by the sea.
Meanwhile, Finland feels exposed as its 1325km border with Russia is almost as long as that of Nato's entire eastern flank. And Sweden sits adjacent to Russia's outposts on the Baltic Sea, with its coastline – especially the island of Gotland – vulnerable to amphibious assault.
Then there's Latvia and Estonia – former Soviet Union states that have been repeated targets of Kremlin threats and espionage.
Estonia's prime minister Kaja Kallas warns the small Nato force currently deployed in the capital of Tallinn could be quickly "wiped off the map" by any Russian attack.
Lithuania's foreign ministry agrees. It says Nato must move away from a strategy of "forward presence", otherwise known as "tripwire" deterrence by punishment, to one of "forward defence" – or deterrence by denial. "We seek that Nato's deterrence and defence adaptation takes into account geographic and geopolitical specificities of our region," a ministry spokesman urged.
Reinforcements
The collapse of the Soviet Union marked the dawn of a new era. The new – nominally democratic – Russia was declared a European "strategic partner".
Things didn't turn out so well.
Putin's troops first marched into the restive Russian region of Chechnya. That was shortly followed by parts of independent Georgia. Then, in 2014, he launched his first assault on Ukraine with the annexation of Crimea.
"Allies – principally but not only Germany – resisted calls over the years to call Russia an adversary," says International Institute for Strategic Studies (IISS) director William Alberque. "This has complicated defence planning at Nato, because how can you make military plans to defend yourself against a partner?"
That's now changed.
"President Putin's war against Ukraine has shattered peace in Europe and has created the greatest security crisis in Europe since the Second World War," Nato chief Jens Stoltenberg declared.
"Now, by referring to it as a threat, this means that the eastern allies have won the argument, and Nato can adjust its plans and policies accordingly to defend against what, in reality, is Nato's main threat," adds Alberque.
Member states attending the Madrid Nato summit this week agreed to mobilise extra forces, raising its rapid reaction force from 40,000 troops to 300,000.
US President Joe Biden committed a further US$450 million worth of rocket launchers and ammunition to Ukraine's defence. He also announced a boost to the US military presence in Europe.
Many of the new troops will form a new military garrison in Poland – the first such permanent facility in a former Soviet Union state. Other US military units will cycle through Romania and Nato's Nordic members. Two extra F-35 squadrons will be deployed to the United Kingdom. Two more destroyers will join those already based in Spain.
Deterrence deterred
"We have reached a turning point," retired German general Hans-Lothar Domroese said shortly after the outbreak of war in Ukraine. "We have China and Russia acting in concert now, boldly challenging the United States for global leadership. In the past, we have been saying deterrence works. Now we have to ask ourselves: is deterrence enough?"
Are tiny "tripwire" forces – backed by nuclear weapons – enough?
"Under the backdrop of what has happened in Ukraine, this is politically suicidal," one senior Baltic defence official told the journal Foreign Policy anonymously.
"Committing yourself to a strategy where you accept that parts of Nato will be occupied, even for a weekend, is a disastrous political strategy. No one can commit to this. So this is something that needs to be changed."
Now a former commander of US Army forces in Europe, Lieutenant General Ben Hodges, and a former Nato director of defence policy, Timo Koster, have added their voices to the debate.
Writing for the Centre for European Policy Analysis (CEPA), they state the West's strategic thinking has been overturned.
"Nato operated for decades on the assumption that its combined military might, underpinned by regular displays of unity and solidarity, would dissuade the bad guys from pursuing territorial ambitions. We were able to deny them success, and punish them if denial failed," they write.
But the global response to attacks on Georgia in 2008 and Ukraine in 2014 emboldened Putin.
"It is clear now that threatening sanctions did not change Putin's mind. It told him he could attack Ukraine without having to face military consequences," they state.
Now Nato has voiced a fear any direct intervention over Ukraine would trigger World War III.
"Putin's bold moves combined with threatening nuclear rhetoric have deterred us from taking decisive action," write Hodges and Koster. "The issue is that a massacre is taking place in Europe, on our doorstep, and that the strongest military alliance in the world is staying out of it. We are deterred and Russia is not."
Reality check
"Finnish and Swedish membership in Nato vastly decreases the chances of war in the east," says Alberque.
Finland has the third-largest artillery force in Europe, behind Russia and Ukraine. It has new F-35 stealth fighters. It also has a well-trained and well-prepared defence force capable of mobilising 200,000 troops.
Sweden brings a modern and advanced navy to Nato's Baltic Sea defences and 207 aircraft.
"[They] have transformed Baltic and Nordic security, reducing the chances of any Russian adventurism to the point of implausibility," Alberque adds. "They would lose, and lose badly if they attempted to approach Estonia, for instance."
Russia's ability to counter the Nato expansion is minimal.
"I say this because firstly, they don't have the troops, frankly, to man substantial new bases in the region, and likely will not have them for some time if this war goes on."
And its capacity to fight such a war is severely in doubt.
Moscow has reactivated retired Cold War tanks to replace catastrophic losses in Ukraine. Combat helicopter attrition has been high. And the expenditure of complicated – and expensive – guided munitions is excessive.
Analysts have been arguing that this proves Russia's military might to be a hollow threat.
But does Putin believe this?
Hodges and Ambassador Koster are not convinced.
"He may even go further, by gambling that a risk-averse alliance would hesitate to defend smaller members, like the Baltic States. He could attempt to cut off the three small states using the significant capabilities amassed in Kaliningrad. So while our leaders do everything to convince ourselves that the Baltics will be defended, the key issue is whether Putin is convinced. And frankly, we don't know."
The conflict has entered its seventh day.