What are you talking about? Russia isn't our ally. And joining NATO would change his agenda. There's no way he believed he'd be accepted into an alliance that's primary mandate is to protect western nations from Russian expansion and aggression. The idea is an absurdity.
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The whole point of letting them in would be to curb their expansion desires. Attacking allies is not exactly cool.
I think you're entirely misunderstanding the circumstances. Putin didn't apply to NATO in ernest, thinking he'd join up with the alliance that's all about opposing him, and suddenly end his ambitions to reconstitute the Soviet Union and continue pushing into Europe. His "attempt" to join NATO was an antagonistic lark. He knew there's no way in hell NATO would have risked giving him access to the alliance's strategic operations plans, and strategy documents outlining how NATO would respond to various actions taken by Russia. Do you really not see how ridiculous such a proposition is?
This is unbelievably naive.