There is no such thing as "NATO expansion". At the end of the WWII, the USSR rolled over the entire Eastern Europe while fighting Germany. After the war ended, they refused to leave, and instead set up Moscow-controlled dictatorships that lasted until the USSR collapsed. I was born in one of such. Couldn't travel nor read foreign books. Didn't have freedom of speech and other most basic rights. An inappropriate joke could get you jailed. I personally knew someone who was sentenced to a Siberian labor camp in the 1980s for advocating human rights. Economy stagnated severely and shops were empty all the time. People had money, but there was nothing to buy in stores. Even top scientists and engineers had to grow their own food on small plots to feed their families. Bananas and jeans were considered rare luxury items.

The shithole finally collapsed in 1991 and Russia was forced to end their military occupation: pack up their dictatorship, dismantle the oppressive secret services and remove their armies. We were finally able to live as free Europeans again after 50 years. Visit other countries, read and write whatever we wanted.

And to ensure that we would never see the return of Russia, we tried to get as deeply integrated with the rest of Europe as possible. That means membership in the Council of Europe, European Union, and NATO.

Eastern Europe's entry into NATO and other organization was motivated purely by the extremely traumatic post-WWII experience of living under the Russian boot. It was our strong initiative desire to join the organization, against cold reception of existing members who had no interest in extending their defence pact. Ironically, they accepted us only because they believed our fears to be unfounded, meaning that they would never actually have to help us fight Russian invasions onto our soil, but saw value in the stabilizing effect that being in NATO would have.

NATO is not a threat to Russia, because NATO mandates civilian oversight over military affairs, and the mutual defense guarantee acts as a moderator that forces every member to consult with others before any military actions. This makes NATO countries extremely stable and predictable. We can see this clearly with aiding Ukraine, where every step is discussed for months and telegraphed long in advance. The over-reliance on stronger members of the organization, namely the US, has led to most European members to being unable to defend even their own country. Who in their right mind calls such countries a threat?

Russians unaffiliated with the dictator Putin understand this perfectly. The man who was Russia's foreign minister for most of the 1990s decribes NATO as "providing free-of-charge security along Russia's western borders". They have no problem with NATO and they are remarkably supportive of it.

The only ones raging over NATO are Putin and his cronies who have destroyed all opposition and consolidated all power in Russia and have now set their sights on external expansion through conquest. NATO stands in their way. They can't invade small countries one by one, but must face the entire bloc at once. They are unable to handle such a confrontation, hence their impotent rage.

The narrative about NATO being a threat to Russia is also ridiculous in how detached it is from reality. If NATO were truly a threat to Russia, then why aren't Russians building air raid shelters as a mandatory part of every building, preparing their bridges for demolition, and carrying out other preparatory work like the Finns are doing? Why are the Finns digging anti-tank ditches and mapping border areas for minefields, while Russia's side is completely open and doesn't even have a chainlink fence? Where is the fear of NATO in real observable facts?

The problem Russia has with NATO is not an imagined threat, but the fact that NATO stands in the way of Russia conquering Eastern Europe again.

If Finland were not allied with anyone, Russia would face only Finnish regular forces with limited foreign aid when invading the country. The maximum opposing force would be relatively small and predictable. With NATO, Russia faces potentially everything, up to American carrier groups and nuclear missiles tucked between cornfields in Ohio. For Russia, demanding that European countries be kicked out of NATO means significantly lowering the cost of invading them, but does nothing to shelter them from the most powerful NATO countries, like the US, which constitute the overwhelming majority of NATO's military force. This too reveals where the true concern lies.

Putin's dictatorship is indeed extremely irritated by Eastern Europe being in NATO for rational reasons, but those are different reasons than you are trying to present.

Besides that I can think of several reasons why Russia only has fields. Firstly, Ukraine is not yet in NATO, so any defensive measures would be premature. And they invaded Ukraine to maintain that state which seems a way stronger reaction then constructing defensive lines. Also building defensive lines implies worries about a ground invasion and it is not obvious to me that this would be Russia's primary concern.

They might, for example, be mainly worried about the nuclear balance. If the USA could place ABM defense systems closer to Russia, they might be able to more efficiently intercept Russian ICBMs in the boost phase when the missiles are still relatively slow and vulnerable. That would diminish the Russian retaliatory strike threat in case of a US nuclear attack. Also having weapons closer to the target reduces the reaction time the enemy has. For that kind of risk ground defense lines it not the reaction one would expect.

It might also be for simple reasons like a lack of resources. Or Russia might refrain from doing what it does not want NATO to do.

People often point to Napoleon or Hitler and claim that Russia fears an invasion over the flatlands of Eastern Europe. Yet, if we investigate this, we find that Russia's European neighbors are building defensive lines on their side, while Russia is not. This suggests that Russia is not afraid of a ground invasion, but its neighbors are.

Likewise, if you look at a globe, Ukraine is nowhere near the launch sites or flight paths of Russian nuclear missiles headed for the US. The Earth is not flat. Nuclear missiles would be launched from sites near in central and northern Russia, flying north over the pole toward the US - not west over Ukraine. Ukraine is geographically irrelevant to these flight paths because it is too far south.

So again, how does NATO threaten Russia?

There is one far easier explanation: NATO is not a threat to Russian security but a threat to Russian ambitions. NATO stands in the way of Russia invading other countries. If every country in Europe were isolated and didn't cooperate with others, a Russian invasion would face only their regular forces. For example, Estonia has just a few tens of thousands of soldiers, less than 100 artillery guns, a few HIMARSes, and some anti-ship missiles. No tanks, no air force, no navy. That's it - something Russia could clearly defeat. But with NATO, Russia could potentially face everything up to American carrier groups and nuclear missiles when invading Estonia. Managable risk grows into unlimited risk.

This is self-evident from the invasion of Ukraine as well. Without foreign military aid, Ukraine could have been defeated long ago, but with the aid, Russia is stuck fighting for just the first fifth of Ukraine. Foreign military aid made the difference between victory and defeat for Russia in Ukraine, so naturally, they seek to isolate countries they intend to invade from military partnerships.