You say Ukraine has to "stand up against it's aggressor", however, Ukraine has invaded various Russian regions both before and during the current continuation and recently their defense minister said they won't stop "until they get to the Red Square" (the Kremlin). In December they even hit Ryazan and Engels, about 300 miles from the Ukrainian border and there have been terrorist bombings in Moscow by Chechen separatists who have been backed by Ukraine all the way back to the 90s.
If it was just about 'retaining' a region they weren't even properly holding before Russia got involved, why would they keep pushing into Russian cities (which if you look geographically, involves crossing various natural borders)
The conflict is a lot less black and white than what the media tells you. The region has always been a hodgepodge of separatists and both Ukraine and Russia are the big players backed by respectively NATO and China to keep the pot stirring.