I can give you the sense in which I consider this the beginning, even though it is mostly restating what I already said. I think the primary reason for this war is that Putin does not want NATO at his borders. That this would not happen was informally agreed in 1990 but unfortunately did not make it into any official documents. After that each eastward expansion of NATO was seen by Russia as the violation of a promise. When in 2008 Ukraine and Georgia started their way to become NATO members this was the final straw and Putin attacked Georgia and eventually Ukraine.
Sure, we can now argue whether the promise (1990), the first NATO expansion afterwards (1999), the Bucharest declaration that Ukraine and Georgia will become NATO members (2008) or Ukraine changing its stance from remaining unaligned to seeking a NATO membership (2014) should be labeled the beginning of the current situation, but I think it is fair to pick 1990 as this is where the situation that Russia does not want was laid out.
There are most likely also other interest such as oil and gas in Ukraine but I don't think they are the primary reasons, even though they might have tipped the calculations. And I consequently think that this could have been avoided if the Ukraine would not have pursued a NATO membership or if Russia would also have joined.
NATO expansion is definitely one of the primary viewpoints. There are convincing rebuttals against those viewpoints as well though. Geopolitics is messy and chaotic. There probably were mustache twirling strategists hellbent on pursuing an anti-ussr agenda despite the USSR no longer existing. I've never heard anyone argue they are the architects of NATO's expansion. I'm curious. European and african commentators I've been checking have very much emphasized that the states themselves decided to join NATO, that they weren't solicited or coerced by NATO itself. Considering lukashenko implied Moldova is next, it doesn't seem like they were unreasonable to try to pursue NATO as a means of assuring continued sovereignty. I think for most of the general public, the beginning was last week. Put to people working in state departments all around the world, off the record and unpublished, I wonder what their answers would be.
I feel like it changes a lot depending on how you feel about hegemony.
Edit: If north and south Korea were about to unify, but China invaded to reconstitute a north Korean buffer state, where was the beginning and what could have been done? Should the south have refused the north because it didn't want to be seen as expanding? Should the north have? China definitely wouldn't be invaded by a unified Korea, maybe they would argue that how they are positioned in international trade justifies it. I'm trying to think of parallels and this is what I've come up with.
Nobody forced anyone to join NATO, at least as far as I can tell. The problem is Putin does not trust NATO, he does not perceive NATO as a defensive alliance that will do nothing unless you poke it, he perceives NATO as an ever expanding organization that accumulates power and influence for the west and the USA in particular. And while the west argues that sovereign countries can decide on their own whether or not they want to join NATO, Putin made it very clear that he considers this unacceptable.
The most similar situation is in my opinion the Cuban Missile Crisis. The USSR could argue that Cuba is free to decide whether or not they want nuclear missiles stationed in their country, the USA argues that it is an unacceptable security thread to have those missiles so close to their border. What would have happened of the USSR had not backed down? Would the USA have tolerated the missiles or would the USA have attacked Cuba or at least the launch sites? What would the USA do today, say if China agreed with Mexico to place military assets in Mexico?
The US stationed missiles in Turkey and as a response the USSR stationed missiles in Cuba. The crisis ended when the USSR publicly deconstructed their sites and the US agreed to do the same to theirs privately. This all because Kennedy needed to save face and have the USSR be the agressor.
So if NATO would decide to invade Belarus right now, then it might be similar to the Cuban missile crisis. Until then, it is just a gross oversimplification of a conflict that is the result of complex relations between countries.
tangent: I don't know if you have ever seen Noam Chomsky discuss Kennedy's decisions at that point, but I think it is the only topic I've ever seen him visibly furious over.
This [1] is probably not the talk you have seen, at least I would not describe Chomsky as furious in that clip. But I was not aware - maybe I once knew and forgot - that removing the missiles from Turkey was not really a concession but planned anyway because they were obsolete.
I am not sure if you understood my analogy, let me try to be more explicit. The USSR tried to put nuclear missiles next to US borders, the USA did not like this. Ukraine wants to become a NATO member, Putin views this as NATO - maybe as a proxy for the USA - eventually putting military assets - potentially nuclear missiles or whatever he is most concerned about - next to Russian borders, Putin does not like this.
In case of the Cuban Missile Crisis the actions of the USSR were only a response but still not tolerated. You could maybe find an equivalent, something that Russia did first and that then caused Ukraine to seek a NATO membership, but I don't think this is important for the analogy. The equivalent of Russia backing down and not stationing missiles in Cuba for Cuba's protection would be Ukraine not joining NATO.
But I have to admit that I don't get your analogy either, would you mind elaborating?
For me the analogy falls flat, as the cuban missile crisis is a response to something the US did, and the invasion of Ukraine is a response to something that Ukraine did (and not NATO)
NATO did not court Ukraine, instead Ukraine wishes to be a member, the current invasion to prevent it being so ironically a case in point. Finland is now considering joining as well, as a response to Russias agression.
It is clear that Russia does not think to invade Finland as well, as it does not consider it to be a part of its old empire, but it does Ukraine.
So in this case it is the sovereignty of a country that is being challenged, as Russia is trying to strongarm countries into backing off from joining NATO. It is not threatening NATO directly. That is a big difference with the Cuban missile crisis.
In the Cuban missile crisis it is very clear that both sides were pointing arms at each other and coercing members of their respective alliances to comply. Here a potential member is invaded. Korea, Vietnam or even the colonisation if Africa would make more sense as an analogy, of power blocs deciding what a country belongs to.
Sure, we can now argue whether the promise (1990), the first NATO expansion afterwards (1999), the Bucharest declaration that Ukraine and Georgia will become NATO members (2008) or Ukraine changing its stance from remaining unaligned to seeking a NATO membership (2014) should be labeled the beginning of the current situation, but I think it is fair to pick 1990 as this is where the situation that Russia does not want was laid out.
There are most likely also other interest such as oil and gas in Ukraine but I don't think they are the primary reasons, even though they might have tipped the calculations. And I consequently think that this could have been avoided if the Ukraine would not have pursued a NATO membership or if Russia would also have joined.