Again I'm not saying I know everything, but if you simply use the argument "they already have Nato on their boarder so there is no difference" its quite obvious you basically have no idea what you talking about and should educate yourself a bit more before getting into further discussions on the subject. If you fail to realize Russia is concerned about Nato expanding to the east and Ukraine becoming a part of Nato, regardless if you think the concerns are valid or not, you must simply have been living under a rock the last 14 years and/or had zero knowledge about the Ukraine situation before Russia invaded them a couple of days ago.
There is also a big difference in importance about how much more vulnerable Russia are along its border to Ukraine, than what they are along the boarder they share with Norway, the Baltics or even USA. Its barely even people living along their border with Norway, its so far up North. Same can be said about their border with Alaska. The Baltics basically have a population of a small Russian city so its nothing they need to be super worried about, from a Russian point of view. But it was quite obvious they were unwilling to let Nato expand to these countries as well at the time, even if they didnt use military force to prevent it. I think this has mainly to do with that Russia didnt have the financial muscles to go to war back then as they have today, but thats just my guess.
Like I said ,I dont think Russia has any right to invade Ukraine regardless if they are concerned about Nato or not. I just find it hard to understand why anyone would think that the reason Russia chooses to invade Ukraine, has anything to do with something else than Ukraines potential Nato expansion.