There is important precedent for a small, determined, patriotic army saving a nation from falling under the sway of Russia. And that precedent is the 105-day Winter War in 1939-40 between Finland and the Soviet Union, the precursor to Vladimir Putin’s Russia.

The courage of the Finns inflicted huge losses on their fearsome adversary, as the Ukrainian army is doing today. Helsinki eventually sacrificed 10% of Finland’s territory to the Soviets, in return for a peace that has endured since the end of World War Two.

To learn the lessons, I travelled to Finland for On Assignment in the days around the anniversary of the end of the Winter War on 13 March. I also wanted to know whether and how this Baltic nation – which shares an 830-mile border with Russia – is adapting to the Russian invasion of Ukraine.

What struck me, in all my interviews, is how the Finns unlike British people  see their national independence as a precious but fragile right, that they do not in any sense take for granted. One manifestation is that 80 per cent of all Finnish men do their military service in the army, and well over a million of them remain part of an army of reservists.

Marko, a paunchy reserve soldier with grown-up children who I met on a snowy shooting range  where he was practising his marksmanship, as he does every weekend  summed up the importance of being in a permanent state of readiness for invasion: ‘A strong army is important because, if there’s an empty place and weak army, there’s a risk someone will come and fill in that empty place.’

There’s a nuance here. Which is that for most of its modern history, Finland has chosen not to provoke the great Russian bear to its east, first by classifying itself since 1945 as a neutral nation, and then – after it joined the European Union in 1995 – as ‘non aligned’.

Importantly, Finland, like its larger neighbour to the west, Sweden, never joined Nato. And for most of its modern history, 80 per cent of Finns were opposed to being part of that western defensive alliance, knowing full well that Moscow views the act of joining Nato by any nation on the periphery of Russia as provocative.

But that policy of uneasily trying to placate Moscow now feels inadequate to a majority of Finns, as they watch the Russian army bomb Ukrainian homes and hospitals. Even as Russian troops massed on the border of Ukraine in the latter months of last year, Finnish public opinion shifted dramatically, and today over 60 per cent of Finns are in favour of joining Nato.

As the senior security and defence official in Finland’s foreign affairs ministry, Kai Sauer, told me: ‘There are some paradoxes in this situation from the Russian point of view. Nato’s popularity in Finland… has spiked.’

Sauer is intimately involved in a fundamental government review of Finnish security, which will shortly be debated by the Finnish parliament – and which is widely seen as likely to lead to Finland making a formal application to join Nato, probably in the late spring or early summer.

‘Now that Russia is busy elsewhere engaged in a huge military operation, this could perhaps really be the momentum to put the application forward,’ Sinikukka Saari, a leading foreign policy expert at the Finnish Institute of International Affairs, told me.

As for the force driving that momentum, it was captured most emotively and aptly by a young Russian student who is studying in Helsinki: ‘In this situation Putin is the Nazi. Despite what they might say on Putin’s propagandist TV, I know it’s a war and I’m against it. I hate what Putin is doing to Ukraine and to Russia.’

The student – who learned her English watching Dr Who as a child in Moscow, and says she tried to imitate David Tennant’s accent – dare not return to Russia while Putin remains in charge. She will be seeking asylum. Her fear about going home is the corollary of Finns’ redoubled determination to strengthen the defences of their home.