Russia's perspective is that NATO (the US) has put military installations in former Soviet satellites (Poland and Romania) and that they're being pushed into a corner as NATO expands to the East. In fact those installations are purely defensive and Russia has become less relevant as a superpower largely because of corruption. This is even more obvious now as they have failed to properly execute a war against their neighbour. Putin and the Russians who support the war are frustrated because they've lost the former glory of the USSR and that Russia is becoming a second rate player in geopolitics. That's why he started this war. He wants the USSR restored. What he will get instead is even more dissent and separatist republics.