Because Europe never invaded Russia through the border at Belarus. They always invade Russia through Ukraine. First Napoleon, then the Third Reich.

Russia was appeasing the fascist West as they expanded their multinational nuclear military without democratic accountability into territories populated with leave-behind armies of fascists that they created. Ukraine was the obvious redline because it is the dominant strategic border, as demonstrated by all European and Russian military strategists in history.

You're confused about history because you don't understand it.