If his goal is to keep NATO off his doorstep he's got a problem because a bunch of countries in NATO border him right now. And if he is capable of running over the country any time he pleases, why has it taken him almost a million casualties and help from another country to keep the territory he has now? The world sees turning to North Korea as an act of desperation-he is not gonna do that unless he feels like he is running out of choices and needs someone else to take the hits in a war of attrition.
As to Russian navy and air force, I think you overestimate their capabilities. We found out in the 90's they were more incompetent than we thought and that was when they were sucking resources and money from the Warsaw pact nations. Yeah, they have some cool toys now, but their tactics and ability to use them are limited. If it weren't for their nukes this would have been over in 2 weeks, only it would have been American and British troops strolling through the streets of Moscow. That fear of Putin saying fluff em if they can't take a joke and launching a nuclear strike is the only thing keeping the west from burying them. Well that and their new allies who absolutely have weapons alot more dangerous than one shiny missile
