It didn't work last time. Last time everyone knew the were Russian soldiers or mercenaries. Ukraine didn't oppose them because a) they held a key road (a bottleneck, really) into Crimea and b) significant parts of the Ukrainian military on the peninsula defected to the Russian side. This time, there's no bottleneck and there will probably be no or only few defectors. This time, it's going to be a shooting war, sadly.