Though since I've not researched deeply, it's a loosely held view I take,
I don't view my below as well read or educated, but it is my view.
. . .
How did it start?
Well, Russia has been expansion focused for a while (I think)
(For the most part)
The Monarchs expanded
The Communists expanded,
The Oligarchs expanded.
(I think)
I vaguely remember something in the timeline about the UN expanding eastward,
Maybe the current leaders saw it as a threat or opportunity.
Lot of mixing of people,
A common excuse by nations to declare war,
By mixing of people, I mean citizens or race being in another country,
Let's say a significant number of Mexicans moved into the USA,
They and or their descendants, could be used as an excuse for Mexico to attack the USA,
If they could claim their 'people are being mistreated, or 'deserve to be under Mexico's government, and since they're already on the land, it makes no sense to force them to move, Mexico will kindly take charge of the land.
I 'Don't mean this example as an attack against immigration or Mexico,
I could have used other nations as examples, USA expansion into Mexico for example,
Or used X and Y instead of real countries,
But I am lazy and don't feel like rewriting.
. . .
But back to Ukraine,
Even before Russia's invasion of Ukraine in 2022,
There was conflict, excuse/reason.
There was that annexation of Crimea,
I recall something about Georgia,
. . .
My point in all this, is that Russia has been expansive for a while,
Soon as The USSR fell,
Power vacuum and people resituating themselves, reclaiming and acquiring power,
When USSR around,
They were acquiring power,
When Monarchs were around, power,
When chieftains were around, I'm sure power.