Russia has occupied Ukraine for far more years than not this past century.
They had literally months to prepare, including the participation of a
puppet state which allowed the Soviets to surround Ukraine north, south
and east.
They had an overwhelmingly superior force by orders of magnitude,
Ukraine practically had no air force, navy and very little armor.

Yet vlad managed to turn this into one of the most humiliating defeats
in history bar none, with 10,000's of dead Russians [and still
counting], the mauling of half his conventional force, and the majority
of the planet turns hard against him.

How did he manage to miscalculate so pathetically ?

Hubris and isolation led Vladimir Putin to misjudge Ukraine

https://www.msn.com/en-us/news/world/hubris-and-isolation-led-vladimir-putin-to-misjudge-ukraine/ar-AAW6KdW
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The truth is incontrovertible. Malice may attack it, ignorance may
deride it, but in the end - There it is !