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He undoubtedly miscalculated. But he is playing the long game. He can afford to as he knows we won't like our way of life affected. And western governments won't like the effect it is and will have on prices of oil/gas food ect ect. So the west don't have the stomach for the long game as he doesIt is not about losing nerve, but having little appetite, and it is a gamble.
Putin is not certainly not stupid, but is this clever? No, he has limited options, and is effectively railroaded. Had Putin foreseen a multi year campaign of this scale he would have prepared better.