RealLifeLore has an excellent analysis of the reasons for Russia's invasion [1], including the military motivations. I highly recommend watching it. In particular the reasoning around gas and oil reserves in the Black Sea have not been given the due attention in the media IMO. He doesn't talk about Finland, but I would imagine the fact that it would broaden the front for a land-based invasion is what motivates Russia's opposition. That and generally having gotten away with being a bully and getting what you want for so long.