The Russians were arrogant. They never, it appears, took into account that the Ukrainians have been planning since their independence for the day that Russia would invade. The Ukrainians knew exactly where every choke point was that could be destroyed and cut off supplies. They repeatedly took the Russians by surprise, often turning them into sitting ducks. When the Russians invaded Georgia they ran over Georgia. They assumed it would go that way in Ukraine. And they've lost tens of thousands of men because of it.