You have to look at Putin's personal history. (This is from memory but I think broadly correct) He was a fairly low ranking KGB officer in Dresden when the wall came down. He advocated a violent response to stop the collapse of the Warsaw Pact but there were no instructions from a paralysed Moscow and consequently with the structures of an authoritarian regime nobody on the ground would do anything hence the troops watching as the wall was torn down. He felt humiliated by what he saw and the collapse of the Soviet Union which he believed in. There were at the time something like 100,000 Soviet troops stationed in East Germany entirely legally under the WW2 surrender terms. Germany wanted to unify with the fall of the wall and Clinton negotiated with Yeltsin. Clinton came to a "gentleman's agreement" that in return for the withdrawal of Soviet troops, NATO would not expand into Eastern Europe. As Yeltsin drifted into alcoholism and Clinton left office nothing was ever signed and the US led NATO reneged on the agreement. Hence why Russia and specifically Putin feels and acts like he does about NATO expansion. I don't believe he intends to invade Ukraine, it would be enormously costly in Russian (and Ukrainian) lives and he could not hold it even if he won, His best chance is to install either through influence or a coup d'etat a puppet government. That said the people of Ukraine are pretty much ideologically opposed to Russia and it would probably be another Afghanistan - a long drawn out guerrilla conflict fought over many years and ultimately fruitless.
Watch "When The Wind Blows" it'll cheer you right up...