It's not a question of his intelligence, it's a land/country grab. Why would he bother invading it conventionally just to turn it into an inhospitable wasteland? He could have done that by dropping a few bombs, that's not what he's after.
It makes absolutely no sense, you're getting overexcited because you've seen 'captured Chernobyl' and jumped straight to some sort of plan of turning Europe radioactive, rather than the more simple explanation - it's the easiest route to get his tanks into the capital.