However, using blunt military force is not a way to present a territorial claim since 1945, among civilized countries. Especially after signing an agreement guaranteeing territorial integrity in exchange for nuclear weapons (Ukraine had produced and deployed most of Soviet ICBMs while it was a part of the USSR).
Whatever legitimacy Russia might have had over Sevastopol and Crimea, now lost forever. I, Russian citizen, can't go in Crimea anymore, because it amounts to visiting the occupied territory.
Some cite Israel of doing the same. Well, Israel gained Golan Heights / West Bank in response to all-out full destruction war against her (and is still being pressed quite hard by international community for it). Ukraine didn't attack Russia in any way.
Not to pass judgement on the current Russian claim to Crimea, but it's a very short-sighted view to pretend that Crimea won't ever be Russian.