Click on the 'Listen' button above to hear this interview.
In 2014, Russia invaded and annexed Crimea, an area claimed by the Ukraine. Since then, violent clashes in Ukraine between pro-European activists and pro-Russian supporters have quieted down - until now.
On Wednesday, Russia accused Ukraine of launching a military attack. Ukraine denied this, and ordered its troops to be on the "highest level of combat readiness."
Christopher Miller has been embedded on the frontlines with Ukrainian troops for Radio Free Europe/Radio Liberty. He says that, with 40,000 troops mobilized on the Crimea-Ukraine border, the calm of the last few years appears to be over. Click on the 'Listen' button above to hear our full interview with Miller.