It's a little more nuanced than "they weren't able to defend their Eastern border". They had to balance between:

- Too little force, which would allow the separatists to de-facto control the territories and lead to a Crimea situation.

- Too much force, which would lead to a Russia invasion triggered by either: Russians dying in Ukraine (since some Russian soldiers were in the region "on vacation") or conflict on the Ukrainian-Russian border (either accidental, or false flag).

I'm just trying to figure out what Ukraine can do aside from let itself get gobbled up. Holding its ground while the economic sanctions/protracted warfare eats into Russian morale might work, or it might take too long. Not sure what else will work though.