Putin would have factored in the threat of the use of nuclear weapons into his plans, prepared long before Lzz Truss got anywhere near the Foreign Office.
This invasion is not a kneejerk reaction to Putin not wanting Uktaine to join NATO or the EC, It is a long term plan by Putin to bring back into Russia, an area of land that was central to the formation of Russian Christianity and which he has always seen as an integral part of the Russian state, not a separate country.
In photographs Putin looks puffy and unwell and rumours abound abotu what this is. I think, whether temporary or disabling, Putin is realising that like everyone else he is mortal and that his time in power may soon end for health reasons.
He is also, it is said, obsessed with what his place in history will be. Currently it is distinctively negative, even from the Russian viewpoint. In his eyes, if he can once again bring back Ukraine and make it, not a separate country, but part of Greater Russia, this will make everything else he has done as nothing and he will be a great Russian hero.