" i cannot fathom what he was thinking "
In fact we know exactly what he was thinking,because RIA Novosti helpfully, is probably accidentally, published an article two says after the invasion which began thus:
"A new world is being born before our eyes. Russia's military operation in Ukraine has ushered in a new era - and in three dimensions at once. And of course, in the fourth, internal Russian. Here begins a new period both in ideology and in the very model of our socio-economic system - but this is worth talking about separately a little later.
Russia is restoring its unity - the tragedy of 1991, this terrible catastrophe in our history, its unnatural dislocation, has been overcome. Yes, at a great cost, yes, through the tragic events of a virtual civil war, because now brothers, separated by belonging to the Russian and Ukrainian armies, are still shooting at each other, but there will be no more Ukraine as anti-Russia.
Vladimir Putin took upon himself - without a drop of exaggeration - historical responsibility, deciding not to leave the solution of the Ukrainian question to future generations. After all, the need to solve it would always remain the main problem for Russia - for two key reasons. And the issue of national security, that is, the creation of anti-Russia from Ukraine and an outpost for the West to put pressure on us, is only the second most important among them."