Initially, the Ukranian army had a hige numerical advantage, we’re talking about having multiple times the numbers of the Russians. Even with the Donezk and Lughansk militias, the russian korps thus always was stretched thin. That’s what lead to that Ukraninan counter offensive. Russia basically bluffed that it had sufficiently fortified both locations that were attacked. Ukraine realized, that could not be the case, at least one would be virtually undefended. And threw thousands at both positions. In the one that actually was fortified, the Ukraninan soldiers were mowed down. In the other, the few russian soldiers pulled back under artillery cover, suffering only light casualties. Ukraine still lost around 4000 men there, but was able to reoccupy the area.

Their idea was right and they got their victory, but a very costly one.