You will notice that when Russia invaded Ukraine they took pains to disguise the fact that that is what they were actually doing: they limited their engagement to levels small enough to be explained away as contingents of patriotic volunteers ("little green men": https://en.wikipedia.org/wiki/Little_green_men_%282014_Crime...) who rushed to Ukraine to support the pro-Russian government that had been turned out in the 2014 revolution.
This was almost certainly in order to make the intervention appear ambiguous enough to avoid provoking a NATO reaction. In the absence of a countervailing power, it's likely the invasion would have looked less tentative and more like Hungary 1956 (https://en.wikipedia.org/wiki/Hungarian_Revolution_of_1956) or Czechoslovakia 1968 (https://en.wikipedia.org/wiki/Warsaw_Pact_invasion_of_Czecho...).