"...while he went off to fight with the rebel Ukrainian National Republican Army"

The Ukrainian National Republican Army was made up of various armed volunteer units during the struggle for independence from 1917-1920. 

The UNR Army had no permanent base and was never able to complete its enrollment, and therefore was not an entirely organised army. 

The UNR Army and the Ukrainian Galacian Army fought together against the Bolsheviks and the Whites.