It may seem that the response of the U.S. and European Union to Russian President Vladimir Putin's Soviet revanchism is entirely ad hoc and reactive. There is, however, a logic to it that even Putin appears to accept, if not acknowledge, which is that Europe has in effect been divided into three castes.
The lowest of these castes, as seen from the capitals of Western Europe, consists of countries in the former Soviet Union's southern underbelly. If Russia attacks or bullies these, the only reaction is dismayed noises.
This was the case, for example, when Russian troops invaded Georgia, in 2008. Similarly when Armenia, under intense Russian pressure, turned down an association agreement with the EU in 2013, there was no visible EU attempt to push back. So the ex-Soviet countries in Central Asia and the Southern Caucasus appear in practice to be recognized as part of Russia's sphere of influence.