Diggers and cement mixers will soon roll into Estonia's fields to give Nato's eastern border with Russia a significant military upgrade. Hundreds of reinforced bunkers will be built as part of a new defensive line to protect the Baltic states — and by extension the entire western defence alliance — from a Russian attack.

Further south, Lithuania is opening more than a dozen so-called counter-mobility parks, stores for equipment such as the anti-tank obstacles, barbed wire and concrete blocks that are all designed to slow down potential invaders. Latvia, like the other two Baltic states, and Finland have also put up fences on their borders with Russia or Belarus.

The works are a visible sign of how security in Nato's frontline states is now determined by Russia's war in Ukraine. Russian forces thwarted Kyiv's counteroffensive last summer and have regained the initiative on the battlefield. Baltic leaders who saw Russia's defeat in Ukraine as the best way of guaranteeing their own security now see the tide of the war turning in Moscow's direction.

"Ukraine is existential right now for us," says Žygimantas Pavilionis, head of the foreign affairs committee of Lithuania's parliament.

Recent months have brought a flurry of warnings about a possible Russian attack on Nato in the coming decade. What is striking is that it is not only Vilnius, Riga and Tallinn that have spread the alarm, but also ministers in Stockholm, Berlin and London, all suggesting a possible confrontation within two to eight years.

Regional leaders see the three Baltic states — with their small landmasses and narrow ground link to the rest of Nato — as the place where an emboldened Putin could seek to test the alliance's unity and resolve through destabilising provocations or even an outright military attack.

Those concerns are heightened by the prospect of Donald Trump returning to the White House after November's presidential elections, which would raise serious doubts about the US commitment to European security at a time when few officials or analysts believe in Europe's ability to defend itself.

"The nightmare scenario would be for Russia, should they be very successful in the near term...to use the opportunity of a Trump presidency to show Nato is completely divided. The place to do that is in the Baltic states," says Jānis Kažociņš, Latvia's former national security adviser and ex-head of its external intelligence service.

Alongside the growing nervousness is a sense that the Baltic states are safer than they have been for centuries thanks to their membership of Nato. It is a "paradox", admits Ingrida Šimonytė, Lithuania's prime minister. The western defence alliance has been bolstered in the past year by the accession of Finland and Sweden, turning the Baltic Sea into what Šimonytė calls a "Nato lake".

Nato countries led by Germany, the US, the UK and others are sending extra troops to the Baltics and other frontline states. Estonia, Latvia and Lithuania are all set to significantly increase spending to 3 per cent of gross domestic product on defence in the next few years, well above Nato's 2 per cent target, which Trump has suggested could become a requirement for US protection. At the same time, Russia is bogged down in Ukraine and its western border areas facing the Baltics and Finland have been all but emptied of troops.

"We are more secure than ever before," says Margus Tsahkna, Estonia's foreign minister. "If Putin tests Nato, Nato will work."

Tsahkna's comments underscore the fine lines that senior officials, diplomats and experts in the Baltic region have to tread when addressing what they see as a renewed threat: raising the alarm without sounding alarmist; urging fellow Europeans to invest more in defence without undermining deterrence; and preserving confidence in collective defence without being complacent about US isolationism.

"How do you not cry wolf but at the same time draw attention to your existential issue?" asks Helga Kalm, director of the Lennart Meri Conference, an annual regional security forum.

Estonia, Latvia and Lithuania joined Nato almost exactly 20 years ago, weeks before they became members of the EU. Annexed by the Soviet Union in 1944, the three nations regained their independence in the early 1990s. Their frequent warnings about Russian revanchism were ignored — a mistake acknowledged by France's Emmanuel Macron and other European leaders.

This time, the threat assessment is more widely shared by allies, but not necessarily with the same sense of urgency, says a Baltic security official.

Security officials apply a standard model to assess the Russian menace: whether it has intent, capability and opportunity. Across the Baltic states, Russian intent is permanently assumed. But it is hardly specific. Estonia's foreign intelligence service said in February that "Russians in their own thinking are calculating that military conflict with Nato is possible in the next decade".

On capabilities, there is widespread agreement that having diverted all resources to Ukraine, Russia is in no position to attack Nato any time soon.

"They really understand that attacking Nato is impossible," says Edgars Rinkēvičs, Latvia's president. "What they are hoping for is that we get weaker. They will do whatever they can to attack us psychologically."

Russian military reform plans unveiled last year envisage doubling the number of troops stationed near the Baltic region to almost 40,000 by 2026 with a big increase in tanks and armoured vehicles. Russia's focus on Ukraine makes the timetable hard to meet, the security official says. But, crucially, once the assumption of confrontation is made in the Kremlin, the Russian military will ensure it is prepared.

With defence spending running at about 6-7 per cent of GDP, Russia has cranked up its defence industrial machine, turning out 4mn artillery shells a year as well as hundreds of tanks and armoured vehicles. Much of it is low-technology equipment for a low-tech war, but Russian factories are vastly outproducing the west. Furthermore, having switched to a war economy, the Kremlin may be unable to switch back.

"It's not a question of whether [Russia] wants to stop, but it's a question of whether it can stop," says Šimonytė. "Because when you've put all your bets on the military economy, you have millions of people who are armed and trained. And, all of a sudden, you say: 'Now, we go home.' I mean, it might very well mean the economic model will collapse...So I think this is dangerous."

That leaves the opportunity. Latvia's Kažociņš sees two possible scenarios in the near term. In the first, Russia is successful in Ukraine and tries to exploit a weakened west. "The way to do that is a quick coup de main [surprise attack] in the Baltic states and follow it up with a credible nuclear threat," he says.

The second scenario would be if Ukraine and western unity hold, then Russia could try diversionary provocations short of a full military attack — with dozens of possibilities for hybrid attacks from sabotage and assassinations to stirring up local Russian-speaking populations. "In terms of scenarios, I could present 100," says Vaidotas Urbelis, policy director at Lithuania's ministry of defence.

A senior EU diplomat in the Baltics adds: "The perception here is that the Baltic states are at risk. In a multi-formed way: not just militarily, it's society, it's hybrid, it's the economy."

Russian-speaking minorities make up about a quarter of the populations of Estonia and Latvia. Baltic officials concede that they are vulnerable to disinformation through Russian media, but say that Moscow's ability to weaponise them is diminishing as older generations die out and younger people become more integrated. "It has changed tremendously, especially with this younger generation," says Alar Karis, Estonia's president.

Lithuania has a far smaller Russian-speaking minority but it is vulnerable because of geography. It borders both the heavily fortified Russian exclave of Kaliningrad, which Lithuanian intelligence believes houses nuclear weapons, and Belarus, now firmly under Kremlin control. Then there is the narrow strip of land that constitutes the Lithuanian-Polish border, known as the Suwałki Gap. For many military experts, it is the most vulnerable part of Nato's entire territory. Deterring Russia here is about denying it the opportunity to exploit any of the Baltics' geographical weaknesses.

"It's the wet dream of Putin to test Article 5," says Rihards Kols, head of the Latvian parliament's foreign affairs committee, referring to Nato's mutual defence clause. Kols adds that Putin longs to "abolish Nato, to be the leader who really challenges Nato, and it falls apart. Any deterrence [by Nato] should be by denial."

As recently as a few years ago, Nato's plan to defend the three Baltic countries was to allow Russia to first occupy them, before repelling its military with mass force several months later. Today, the strategy is to defend Nato territory "from the first metre" and not let Russian invaders raze cities as they have in Ukraine. Defence by denial means making it prohibitively difficult and costly for Russia to attack.

But the defence alliance is gradually stepping up its presence in the Baltics. Nato has conducted air-policing missions over the three countries since they joined in 2004. In 2016, following Russia's annexation of Crimea, it added multinational battalions and battle groups of troops. Now, it is increasing those battalions of about 1,000 troops each to brigades of 3,000-5,000 troops. Those forces would be expected to engage Russia in any attack until reinforcements arrived, potentially by sea following the entry of Sweden and Finland into Nato.

"Our understanding is simple: when Russia looks at us, they look at capabilities that are here now, today. Russia thinks they could delay and disrupt reinforcements. Deterrence isn't stable if it's based on only reinforcements," says Urbelis, the Lithuanian defence policy director.

However, for the Baltics, the crucial guarantor of Nato strength remains the US. Keeping the US engaged in Nato and also in Ukraine is a top diplomatic priority for Baltic leaders and politicians.

Many Europeans are deeply concerned about what a second Trump presidency may bring, especially after his comments that Russia could do "whatever the hell they want" with countries that do not spend enough on defence. But none of the Baltic leaders or officials interviewed say they needed a Trump contingency plan. "We are not that worried. We rather see it as an opportunity to push Europe to spend more [on defence]," says one senior Baltic defence official.

A commonly held view in the region is that Republican presidents have been better for Baltic security than Democrats. Trump's first presidency "from a regional perspective was very much beneficial", says Latvia's Kols.

Lithuania is hoping that its close ties with Taiwan and antagonistic relationship with Beijing will earn it credit in a second Trump presidency. Kažociņš, Latvia's former national security adviser, says the US will need to show fealty to its allies — and prove it can be depended on — to win their support on China. "The US will not be able to deal with China on its own. It will need allies. The US medium-term interests go directly through Kyiv and Article 5," he adds.

Latvian President Rinkēvičs says consideration of America's role in the world went beyond Trump and his Republican allies. "We see some profound change in thinking in the US," he says. "It is not about the current political cycle. It goes deeper."

Like his Baltic counterparts, he shares the broad scepticism over the ability of European powers to step up and replace the US. Although criticised for its reluctance to send long-range weaponry to Ukraine, Baltic officials say they expect Germany to make the biggest contribution to beefing up Nato's frontline defences since Berlin was focusing its rearmament efforts on the army and on land warfare. By committing to base troops in Lithuania — its first permanent overseas deployment since the second world war — it is putting its forces directly on the frontline.

France and Britain are seen as more thinly spread. French President Macron's hint this year that he could send combat troops to Ukraine won plaudits in the Baltic region but there are doubts about whether Paris's actions live up to its rhetoric. Frontline states would like Britain to take more of a leadership role but are unsure it will invest enough in its military to do so.

"The US is still the only game in town," says one Baltic security official.

The paradox of Baltic security and how Nato allies respond to its warnings means the mood in the region is prone to sudden changes of sentiment.

Tomas Jermalavičius, a research fellow at the International Centre for Defence and Security think-tank in Tallinn, says the Baltic region feels particularly vulnerable right now. There is a sense that the two premises of its post-independence security policy — never again to being occupied and never again to fighting alone — are under acute pressure.

First, the scale of Russia's onslaught on Ukraine shows that allied forces deployed to the region are insufficient to deter Moscow and the west is too slow to respond to aggression. Second, the Ukraine war has brought into sharp focus the continuing divergence between frontline states and Nato members further west on the true nature of the Russian threat.

"Once the alliance and the allies pull themselves together — which they are in the process of doing so in many regards — I think we will emerge from that window with less trepidation," Jermalavičius says.

Not all frontline states are so concerned. Alexander Stubb, Finland's new president, told the Financial Times this month that he was not worried about a Russian attack and that he was alarmed by "this rather belligerent talk" about Moscow testing Nato.

Officials in Finland, which fought a ferocious war with the Soviet Union in 1939-40 but kept its independence, underscore however that their experience is different from the Baltics, which were illegally annexed by Moscow after the second world war.

Throughout the Baltics there is consensus that Nato allies must now step up their commitments to collective defence. "It is very much in Nato hands to deter: by having troop deployments to defend territory from the first metre, realistic defence plans, overall defence resources. Russia factors these into its calculations. Russia would not look to start a war when the outcome for them is uncertain," says a senior Baltic official.

Lithuanian Prime Minister Šimonytė says Nato members can no longer afford to dismiss warnings from Baltic states as histrionics. "Are we a dramatic, paranoid one-issue country? Now this issue is everyone's issue. It is a bad irony of life."