The Ukrainian military is loath to get involved in a civil war and Russians are fellow Slavs.
If its a war, there has been so far no loss of life and no bloodshed. But things can quickly change without warning.
The Russians sure had no problem deliberately starving 7m rebellious Ukrainians to death during the Holodomor, in addition to conducting a dozen other large scale massacres of Ukrainians in the process of putting down Ukrainian insurrections. Ultimately, the Ukrainians have to decide what their response will be. We can't do their fighting for them. If their response is to fight the Russians to the last dead GI, the way many of our existing allies seem to think, then they should get used to being a Russian province.