They won't assimilate. Russia tried to force them to assimilate at gunpoint for hundreds of years. Murder them, starve them, threaten them, torture them, beat them, imprison them, rape their women, steal their food, kill their farm animals so they can't feed themselves, forbid them from speaking their own language or printing books in their own language, forbid any class from being taught in Ukrainian inside Ukraine....all that has been done by a succession of Czars, then the Bolsheviks, then Stalin and a long sad list of brutal dictators after him.
People who don't know the history act "surprised" by the Ukrainian resistance and fighting spirit. It is not surprising at all to me. Just look at how many Ukrainian celebrities, many with the money and fame to escape to a safer place, have stayed to fight. Pasha Lee, A famous Ukrainian actor who was in The Hobbit, was just killed fighting the Russians when they shelled Irpin. This goes very deep and is a long, seething anger for Ukrainians. And by killing their women and children and senior citizens, Putin has already pushed it far past the point of no return for them. They will never forget this, because now it isn't a story told by grandma around the kitchen table. Now it is burned in the minds of every Ukrainian forever.