1) one requirement for NATO admission, as I understand it, is that you don't have any active wars going on. 2) while there was one case where Turkey and Greece

1) one requirement for NATO admission, as I understand it, is that you don't have any active wars going on. 2) while there was one case where Turkey and Greece went to war when they were both NATO members, generally speaking it is a requirement that you don't attack other NATO members. 3) it is expected that all NATO members must be democracies, and while Russia has had elections, and I think arguably was a real democracy in Yeltsin's day, it hasn't had competitive elections in decades. You don't have to be a perfect democracy to be in NATO (otherwise Turkey and arguably the US might not qualify), but you do have to approximately speaking have elections in which the incumbent might possibly lose (true in both Turkey and US today, not true in Russia). 4) What Putin actually wanted was to reassert Russia's dominance over Ukraine, making it somewhat similar to Belorussia in that it would have a separate government, but would not be really independent. The stuff about worries about Russian security was a pretext, kind of like WMD's were the pretext for the US in the Iraq War but not the real primary reason.