There's an analysis of what happened at the battle of Ilovaysk. The Ukrainains were on the cusp of encircling Donetsk and would have forced the 'separatists' to surrender or be destroyed within a couple weeks, even with has badly led and equipped and undertrained they were. The Russian attack was devastating. The Ukrainian military is sorting through what happened and wrote up their analysis. That analysis is in Ukrainian, so run it through google translate.