The major historical lesson, taught by Russia itself to Napoleon and Hitler, has not likely been forgotten. Putin is smart enough to know that it would apply to a westward invasion as well as it did to the eastward. They have already recovered their (historically) Crimean Peninsula, and the northwestern border area, populated primarily by Russians. What else does Ukraine have that’s worth “civilizational risk” to the Russians?
Suppose Putin's client is not his own conception of Russian national security, or even public opinion at home but suppose his client is China. Why China? Because China is an enormous threat to Russia and therefore it is understandable that Putin wants to make an ally rather than an invader of China. China offers a huge market for Russian minerals and energy as well as technology trade-offs.
He might believe that China is the wave of the future. He might want to forestall China encroaching on Siberia with its population to simply absorbed Rushias mineral wealth. He might just hold a grudge against the West and especially against America- a little man's inferiority complex that puts a chip on his shoulder. Whatever the motivation, even if it is not in Putin's direct interest as we would calculate it on his behalf, as prudent planners we must assume the worst.
A new Russian dictator, Putin, may also have concluded that the West simply would not stand up to China and he had better pick a side while the picking was good.