[Putin took Crimea away from the Ukraine. But Crimea was historically part of Russia, and not the Ukraine (the Supreme Soviet of the USSR simply gave Crimea to the Ukraine in 1954).
And today about 65% of Crimeans are Russian. Only about 15% are Ukrainian.
So while its important for the US - and the world - to stand up to Putin, Crimea is probably not the place to do it. Theres no obvious crime here.
Now, on the other hand, if Putin should move into the Baltic States...]
Territorial claims are often a tough call. Not that anyone asked me, but I look at it this way. One, is there validity to the claim? And two, is it worth getting the US into a conflict over that claim?
Take Crimea. The Russian claim on that territory is valid enough - in my mind - that it’s not worth getting the US involved. If Turkey or anyone else wants to get involved, that’s their business.
Now take, say, Estonia. As you noted, Estonia was once part of the Russian Empire. But I don’t see a Russian claim to Estonia as valid. For one thing, most people living there are a separate ethnic group.
So if Putin moves on Estonia, I’d take action. Not with tanks (too risky), but with a strong economic blockade, like was done to apartheid South Africa.