The West, (NATO), gave certain assurances about expansion, Putin is a slippery, corrupt fuck but I think he's got a point in this case.
But surely it's up to Ukraine to decide if it wants to join NATO? Should they say no just because Russia doesn't like it?
It definitely feels to me that Putin doesn't feel he can lose here. If he invades and NATO doesn't act, he gets Ukraine and makes the West look toothless. If NATO does he'll use it as an excuse that Russia is being bullied and he has the right to take action.
Then there will be China's actions. If the West is busy dealing with Russia, they'll use it as an opportune time to seize total control of places like Taiwan, Hong Kong and Nepal.
There is the chance that it could escalate quickly and with the likes of Johnson and Biden in charge it hardly fills you with hope (though at least it's not Trump)