To prevent the Ukraine from being invaded at all -- never mind who wins -- the Central Powers have to hold out longer. The Communists weren't going to take on the Germans after their humiliation in 1918. That means that Skorpopadsky lasts longer, too. He has to use the breathing space wisely, and reach an accommodation with the nationalists and find outside allies who are able and willing to help keep the Ukraine independent. Maybe Turkey after 1923?
A couple of minor points: the Communist state wasn't called the USSR until 1922. Previously it was called the RSFSR (=Russian Soviet Federative Socialist Republic).
France did have forces in the Ukraine (specifically, Odessa) from Dec. 1918 to April 1919, but the army was no real help to the Whites, and the navy crews mutinied.