From the Russian point of view that's your mortal enemy that has expanded for the past 20 year and now it's knocking on your borders. How would the US react were Canada, an "independent, democratic country", "voluntarily joining a defence-alliance" with Russia that allowed Russian military to place missiles right on the US border?
It will be a cold war situation. If one side brings nukes to the border, the other side will do so as well. A possible deal would be something like NATO and Russia removes their nukes from border countries. And maybe make another deal to remove the amount of nuclear warheads.
