There are a lot of speculations about why Putin decided to invade Ukraine.
In ethology it is a game that explains this:
It is called the game of hawk and dove and it applies to the bird's territorial conflicts .
Imagine 2 birds who are fighting for same territory, they can choose 2 behaviours:
-be a dove: avoid the fight, avoid injury, be content with less and survive. Usually dove live with doves and cooperate
-be a hawk: search to maximize the gains, and take aggressive actions that can kill you. ("Greed is good" Gordon Gekko, "Wall street"). The hawks are stopped/killed by other hawks.
It is clear that Kremlin is inhabited by hawks. You don't survive communism and wild capitalism be being a pussy.
On the other side the situation is more complex:
Let's define other side: Ukraine, EU and USA.
In birds ethology naturalists observed a 3-th behaviour: to be a bourgeois: if you are attacked you defend the territory, you don't enter in other territory. The bourgeois fights off a hawk. This is Ukraine.
EU, in particular Austria,Germany and France behaved as a dove with Kremlin, seeking always to avoid conflict and compromise. This worked inside Europe, because nobody has to gain something by using aggression.
With respect to Ukraine and Eastern Europe, EU was like a selfish herd, with Western Europe in the middle, and Eastern and Southern Europe at periphery, as a buffer against Russian and Middle Eastern threats.
The corruption of European politicians, the infighting, the narrow interests produced contempt at Kremlin and underestimated Europe. Gerhard Schroeder ex canceler of Germany is still on the payroll of Gazprom.
USA is a hawk and it will be a hawk for a long term.
USA was created by people who behaved hawkish, they crossed the ocean, they killed native americans and took their lands. Also it is protected by 2 oceans, and by biggest and most technically advanced armed force in the world, so the external security risks for USA are minimal. The only attack on american soil after WW2 was on September 11 2001 , and then USA behaved hawkish, chasing the authors of the attacks all over the world and killed them.Other countries didn't take the same actions. For example Spain retreated from Irak after Atocha attacks.
Putin as a hawk saw a bunch of doves and decide to attack. The big bald eagle was oceans away and without support of Europeans it can't help a country isolated from big oceans.
So what went wrong ?
Well, the EU wasn't a dove, it was a bourgeois all the time. It wanted to maintain the status quo, with minimum effort, so it made concessions.
Eastern Europe was traumatized by Russia and Western Europe by WW2, Putin invasion was felt like threat to all. The politicians that were friendly with Putin felt betrayed.
In order to maintain the status quo, Ukraine had to be helped. This was dictated also by cold state interests, if not by human solidarity.
In this 4 actors game, EU want the status quo to be maintained: to have access to raw materials from Russia and Ukraine. This is impossible for now.
Ukraine wants to maintain its territory, economical and political independence from Moscow. But you have what you can defend. West sends weapons, but Ukraine will have the manpower to man those weapons?
The decisive factor in Ukraine will be manpower, West might have better equipment by the supply chain is long while Russia has shorter supply chain and a competitive equipment. Technologically on the battle field they are balanced.
I think Russia is afraid to declare war on Ukraine and mobilize it's troops, because behind curtains it was sent the message that NATO will intervene.
NATO intervened in Afghanistan which is thousand of kilometres away against an enemy who didn't pose the same threat as Russia, and it will not intervene at it's borders?
If Russia doesn't cooperate and it doesn't attack NATO an incident in Kaliningrad enclave or in Black Sea can be created to convince public opinion that the war is necessary. Or a global famine will make countries plea for NATO intervention in Ukraine to free the wheat.
France and Spain can recruit in their Foreign Legions Africans starved by Putins blockade to fight in Ukraine.
If the current mobilisation is maintained. The conflict will be long. Russia can't push in Ukraine, and Ukraine can't push in Russia.
If Russia mobilise all the reserves, it could lead to WW3. I hear more often on youtube, stuff like: a small nuclear is explosion doesn't damage too muck the environment, after 2 weeks radiation is gone.
USA had neutron bombs ready to use against soviet tanks in Germany who was more densely populated than Donbass.
USA can follow it hawkish strategy for a long time, even if Biden lefts the office, the next president will feed military industrial complex with aid money for Ukraine.
Support for Ukraine means support for Taiwan, if USA waivers in Ukraine, China might risk taking Taiwan, and drag USA in a war, which it will be more costly for USA, in terms in of manpower and money.
China is an self reliant economic giant, with a huge and modern army. The only weakness is oil and food.
How long Russia can hold its hawkish attitude ? Putin and Co cannot retreat from Ukraine now. The empires has to win every battle because they have many enemies that seek for a chink in the armour.
Troubles from northern Ireland lasted from 1960 to 1998. It was was a slow burning conflict localized conflict. It will be the same with Ukraine ?
Or it will be like on WW1 Western front, a trench warfare?
I estimate that the war will take minimum 2-3 years and maximum 10 years.
In 2 years maybe EU will decoupled energetically from Russia and apply more sanctions, and send more weapons. Germany lost in WW1 because it was starved and USA supplied weapons to Entente camp.
EU will be in an economic crisis, but the orders for weapons will flow to keep the economy.
Meanwhile Russia can diversify it's clients selling to China and India or other countries, and EU will have to replace Russian raw materials with those from Africa and Latin America.
Russia can rebuild it's armament industry in 2 years, but the problem is the capable people. Moving the factories from Western Russia beyond Ural mountains in WW2 was a gigantic feat. But Stalin still got engineers, patriotic people who worked hard on the cold, half starved t rebuild factories and NKVD for people who lacked patriotism.
Putin has only NKVD. Smart people left Russia.