There are many things to consider in this game of politics.

1. The territorial intregrity of Ukraine
2. The closness of Crimea and Russia to EU and NATO members.
3. The gas pipes passing trough Ukraine which keep Europe warm during winter providing over 30% of all gas coming into Europe.
4. How much do EU or NATO want to back up Ukraine.

Considering all there this I think that EU and NATO will not allow Russia to take Crimea - EU cause it endagers both point 1 and 3 which are important as EU probably plans to add Ukraine in itself with time; NATO will probably be more reserved but Russia on the Crimea would be a bed strategical sitation - due to that allowing the Russian Black Sea fleet to endanger key instalations in Turkey and Greece. Both arleady stated with recaling diplomats, treathing with embargos after which comes starting embargos and then probably a threat of military intervention. If an epic fail or mistake happens and it goes hot, than Russia stands no chance due to the outdated techs that it has and far less manpower to call to arms then NATO and EU.

But in the end it all lays on how far are NATO and EU ready to go in squising Russia out of the peninsula.