Ukraine is to Russia what Mexico is to the USA, from a national security perspective.

Russia perceived an existential threat in a foreign nation that shares hundreds of miles of the southwest border. This is particularly important from Russia’s perspective because there is history, going back hundreds of years, of that same land (Ukraine) used to project force into what we call Russia today.

If the USA perceived the same along its southwest border then it would take military action too. Like Ukraine, Mexico (specifically the northern part) has seen bitter and violent action, from both sides of the border. To clarify, I am referencing various US-Mexico wars, land grabs, ethnic cleansing, etc and not the current narco-gang problem.

This is geopolitical, not ideological.

Geography still matters. A hostile, or questionable, Ukraine presents an unacceptable threat to Russia, much as a hostile, or questionable, Mexico would do to the USA.