So why is Putin invading Ukraine now? There are several logical reasons if you understand Russian history, geography, psychology and demography.
In terms of history, in the past 500 years, Russia has been invaded several times from the west. Specifically, Moscow has been attacked and conquered six times in its history by foreign armies, and usually via the same routes. This is also where geography plays an important role in understanding the Russian invasion.
In terms of geography, the Carpathian Mountains are a natural land defence between the Baltic Sea, and the Black Sea. Indeed, in the days of the USSR, under the Warsaw Pact, the Russians could project military power in the land gaps either side of the Carpathian Mountains on Baltic and Black Seas. This is where NATO expansion has worried the Russians because, post the USSR, I speculate they feel Moscow is exposed, and is being encircled by NATO.
I also speculate the psychology of the Russian leadership is one of paranoia of land invasion, and they want to push out territory (or at least project military power) to fight an invasion from the historical routes Russia has been invaded from. Post-communist Russia has been doing just this and is why Crimea was effectively annexed by Russia (because they wanted to project naval power into the Black Sea).
From a US perspective we like to think that the US was the biggest factor in the Allied win of WWII, but little thought is given to how important the Russians were and how many Russians lost their lives. This is where, most mainstream media has completely omitted the rise of far-right violence and politics in Ukraine. No one in the West wants to think they are supporting Nazis even if they are fending off the Russians. Please note that I am not saying that all Ukrainians are far-right, however I speculate that the Russians do care about the rise of the far right in Ukraine (as it is along their border), and from their perspective see it as stopping the rise of another WWII Nazi-Germany type situation.
Lastly, demography plays a part because Russia has had several 'baby busts', that is not enough babies have been born. I speculate that the Russian leadership believes that if Russia does not plug the geographic routes for invasion now, it never will.
Sadly, I also don’t see Russia stopping with Ukraine. If the Russians will stop at nothing to gain geographic security this puts Russia in a direct confrontation with NATO member states. I also worry about the second order consequences, like the loss of life and suffering that will happen due to a loss of Ukrainian wheat crops, and the loss of global crops due to sanctions on fertilizers from Russia.