The second map is an infographic from NPR. It shows you why other Russian satellite states are nervous about Russia's annexation of Crimea on the basis of ethnic kinship:
The greener the country, the more ethnic Russians live there. As the favored ethnicity in the Soviet Union, Russians were workers, soldiers, managers and party bosses all throughout the Soviet Union. After the fall of the USSR, many of them or their descendants stayed put.
So the pretext that Putin gave in Crimea could be repeated in, say, NATO member Latvia, to "protect" about half a million ethnic Russians there. Already, as NPR points out, "since the Crimean crisis broke out, Transnistria's local Parliament has asked Moscow to grant the breakaway region [of Moldova] Russian citizenship and admission to the Russian Federation." And parts of Kazakhstan are more Russian than Kazakh.
So what is to stop Putin continuing in this way? So far, only Putin himself. That is precisely what worries the West and Putin's neighbors alike.