It was different in the past, now political pressure is more important, as is the fact that the super powers are very different.
Russia doesn't want to be aggressive in Crimea, as if it is it will cause them to lose business and productivity with other countries who will sever ties with them.
On top of that there is always the chance that other countries will come to the defence of Crimea and the Ukraine, but it's unlikely as with today weapons it would all be over very quickly (mainly due to missile technology and nuclear advancements, but also due to many huge advances) back then it was hundreds of thousands...
Now, and even without the use of nuclear weapons, it would be hundreds of millions if WW3 started.
Fortunately not even the Americans are stupid enough to threaten any military action over this, they know that it could spike all out warfare, and that could include nuclear annihilation of Russia, the US and Europe, if not the entire planet.
After WW1 and WW2 the superpowers know a third world war would be absolutely insane!
If it was missile warfare it could all be over within a few days, as both sides will use most of their nukes and most of their nuke counter measures.
P.S. Russia has the most advanced missile tech by a mile http://en.wikipedia.org/wiki/RT-2UTTKh_Topol-M