Russia lost 14 Former Soviet Union (FSU) states after the fall of the Berlin Wall.
Could you imagine the United States or the European Union losing something like that?
Think of California and Texas and more seceding and making their own independent states.
What would that do to the power and capability of this country?
Hence, when Russia goes into FSUs like Georgia and Ukraine (in red)--does a land grab like with Crimea and Abkhazia and South Ossetia--and threatens others, in their Soviet minds, they are just reclaiming what is/was theres.
Now listen, I am not justifying their aggression and hostile actions, but I am realizing/recognizing how explosive a situation this may end up being--especially since Estonia, Latvia, and Lithuania (in blue) changed sides and are now part of NATO!
Russia--even without the 14 FSUs--is the largest country in the world by territory with over 17 million square miles--that is almost the size of the United States and Canada or and China combined!
Russia may not have the economy of the United States but they are a formidable foe that we least not forget also has the largest nuclear weapons stockpile in the world.
Russia has a history of creating a large world dominating union and their military forays into the FSUs, support of hostile regimes like Syria and in Iran, sizable war games in Belarus, continuing to militarize the Arctic, interfering in our election, stealing our secrets via Kaspersky Lab software is likely just a shadow of what is yet to come.
So if you think North Korea and Iran are problems... ;-)
(Source Map: here with attribution to Map Collection)