Quoting Jeffrey Veidlinger’s In the Midst of Civilized Europe: The Pogroms of 1918–1921 and the Onset of the Holocaust, pages 288–289:

In April 1920, Polish and Ukrainian forces advanced toward Kyiv from the north, beginning what came to be known as the Polish–Soviet War. Their goal was to challenge Bolshevik ascendency in the region and reclaim the historic lands of the Polish–Lithuanian Commonwealth.

Petliura, who was then in exile in Warsaw, had agreed to an alliance with Piłsudski in which Poland would support Ukrainian independence in the provinces of the former Russian Empire in exchange for Petliura’s renunciation of Ukrainian claims to Eastern Galicia.¹ Polish forces had already made gains in Belarusian territory, conquering Mazyr and Kalinkavichy in March.

The Red Army was completely unprepared for the assault. Polish soldiers were amazed at the bedraggled appearance of the Bolsheviks. “Some were barefoot, some had shabby soft shoes, others had rubber galoshes,” wrote Franciszek Krzystyniak, “while on their heads they wore a variety of headgear—some even had women’s hats—and winter caps or kerchiefs, and some were even bareheaded, their hair flying in the wind. They looked like ghouls. Their rifles were either suspended on string, or without any straps—but they did have plenty of ammunition in their pockets and their aim was good.”²

The Bolsheviks’ tenuous hold over Ukraine seemed to be evaporating just as it had in the face of the Whites eight months earlier.

(Emphasis added.)

Maybe I should have known this far sooner than I did, but I was always under the impression that the Red Army had simply invaded Poland unprovoked!