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His argument will be that he had to move into Ukraine to stop them joining NATO, in which case NATO would be duty bound to declare war if he moved into Ukraine. There's a logic in there somewhere.
His invasion has directly brought about the strengthening and expansion of the NATO alliance. He's reached his position of power by clearly being a smart / ruthless operator, but totally thrown it all away with this massively misjudged war.
Putin does not regard NATO as a threat. Putin regards NATO as unlikely to intervene directly if he invades Ukraine, and has been proved correct.