Russian President Vladimir Putin has threatened to retaliate against what he called Ukrainian "terror" in Crimea, the peninsula annexed by Russia from its neighbor two years ago. That raises the prospect of a return to all-out war. At the moment, the purported terrorist activities that Putin has vowed to put down are very much in dispute. The region is awash with military personnel and hardware from both nations, as well as the two rebel armies based around the cities of Donetsk and Luhansk.