Indeed, you can drag USA or whoever into this discussion but the solid fact is that the only troops outside of their own borders in this crysis are russian troops. Noone elses. And the other solid fact is that Russia wants Eastern-Ukraine. Cause they already got Crimea and Crimea without Eastern-Ukr is kinda of a shitty win.
russia cannot afford eastern Ukraine, it will be too much strain on their budget, which is cracking as it is with the given oil prices. What russia and putler wants is Ukraine to stay in the same deep-shit situation, unable to move away from russia, however it wanted to. And one way to achieve this is a way russia already tried (and verified) twice: create a frozen conflict. Mind you - the current one is not freezing, but the fact, that the only ones attacking currently are the so called "rebels" speaks volumes on what Ukraine can afford right now and that they would settle with a partial loss of their territory. Not because they want to or do not mind accepting this, but because they cannot start another fight with separatists, who are basically a thin camouflage line on the bumper of russian tank. Stopping that tank would cost too many lives and I guess it will be attempted only in the most extreme scenario, which I think Poroshenko wants to avoid, given that huge losses of life would not be accepted easily by Ukrainians.
Numerous observers noted, that the best Ukraine can do is what Poland and baltics did: i.e. reorganize, reform, root-out majority of corruption, improve conditions of life and economy. THIS is what putler does not want most: busted myth that slav nation cannot live without support from russia. This would hurt his rule more than ANYTHING that Ukrainians can throw, launch or fire at the rebels. Then, when you see Ukrainian shopping malls full of russians buying stuff which is unavailable or overly expensive in russia, when people see that there is a better way to live, than being sheeple under a wolf leader (granted - the herd is big, the chances that YOU will be eaten are acceptably small), THEN the process of "joining" will begin to reverse and russia will implode on the weight of its own bullshit.