This is a REALLY dumb move on the part of Tillerson. And so, by extension, it is a really dumb move on the part of the Trump administration.
Crimea was part of Russia for centuries. Not just for decades, but for centuries. Then in 1954 Khrushchev - on a whim - assigned the Crimea to the Ukraine.
Russia will not “return” the Crimea to the Ukraine. Just as the U.S. will not “return” Virginia to Great Britain.
So now what? Either Tillerson will have to walk back his threat, or the U.S. and Russia will be in a perpetual state of tension. Both of these possibilities are bad.