President Obama spoke on the 28th about reports of Russian troop movements into Crimea, saying:
[We] are now deeply concerned by reports of military movements taken by the Russian Federation inside of Ukraine. Russia has a historic relationship with Ukraine, including cultural and economic ties, and a military facility in Crimea, but any violation of Ukraine's sovereignty and territorial integrity would be deeply destabilizing, which is not in the interest of Ukraine, Russia, or Europe.
By leaving out that the sovereignty and stability of Ukraine are American interests, I'd normally ask if this was read as the Russians as placing Ukraine outside of our defensive perimeter, thus removing any doubt about whether the full occupation could be carried out.
But by this point in the Obama administration, I honestly don't think that Putin even considers our potential reaction when making decisions. Leading from behind is too much for us now, it seems.