The recent situation with Russia and Ukraine has seemed murky to me as soon as Russia moved on the Crimea. Whatever one might think would be the right thing to do, we have done little for the same reason that we did probably nothing to free Tibet from China: too far away and impossible to do without risking a major war.
The idea that there is anything we can do to dissuade Putin from acting in Russia’s historical sphere of influence seems pretty nonsensical. W did, or could have done nothing to keep him from moving on Georgia.
Buchanan is probably right; we should engage with Putin where possible and try diplomacy and strenghtening alliances to contain Russia if they become too ambitious. (Such as installing missile defense systems in places like Poland and the Czech Republic. And we all know what happened to those plans. And we should avoid stupid moves that reopen areas such as the Middle East to renewed Russian influence.)
Realpolitik is not going away anytime soon.