[Brian Swartz]

I don't think it's at all foregone that Russia would not be a threat to mainland Europe. I think it's inevitable that if nobody defends nations like Ukraine, it could very well eventually be one, and even if it's not Russia and mainland Europe, it will happen elsewhere in the world. At some point you either decide to oppose wars of aggression/annexation, or you decide not to do that.

I don't want to quibble over dates, but saying they've only been independent for 10 years or so when in fact it's been 32 years is a rather large difference. The Crimean situation in 2014 is complex; you can make an argument that self-determination ultimately happened there, but Russia deposing the government and occupying it militarily prior to a rushed referendum is not exactly what you'd call and free and fair expression of the will of the people.

There's nothing so muddled about this war, and let's say Ukraine got no support, gave Russia some black eyes early, and ultimately were overrun eventually. Why would they stop there? Barring a much different government in Russia, I see no compelling reason to think they wouldn't keep grabbing as much as they think they can get. Not immediately, but when they thought the time was right. The more territory and resources they control, the more leverage they'd have in the region.