Same sort of thing, except that the Ukrainians did engage in ethnic cleansing.
Both originated in some sense before WWII. Then, during Nazi occupation, they fought the Nazis. This, of course, placed them on the same side as the Red Army, which seemed reasonable when they were both fighting the stronger force. No formal alliance was necessary.
As the Red Army approached, they realized that the Nazis were losing and the Reds were no better, so they turned and starting to fight the advancing Soviet army. In so doing, they then were on the same side as the Nazis, and at that point stopped killing Germans and starting fighting the Soviet Army (when it hove into range) and its underground allies (already present). This took some moxie, considering that the Germans killed a couple million Poles. I suspect the thinking was mostly, "Okay, I'm going to side with the weak against the strong since it doesn't matter who wins, we're royally f--ked--and the stronger the victor, the worse the f--king."
In both cases, there's backstory. Centuries of backstory. Russia was the imperial force and colonizer in Poland for a couple hundred years prior to WWII, and had divvied up Poland as its own special backyard under a Soviet-Nazi cooperation/defense agreement. When Germany took over, the Soviets moved in under the guise of "we're your buddies," rounded up the Polish officer corps and shot them. This, after a brief war when the US was celebrating its victory in WWI where Soviet Russia, the anti-imperialist force, was trying to restore its empire in Poland.
The Ukrainian story wasn't much better, except that Muscovite supremacy over the territory had gone on for longer, and there was the overt, explicit attempt to, in current language and socio-political jargon, cancel and erase the Ukrainians and their history.
Neither movement was especially nice. Both were nationalist in a nasty way, the Ukrainians engaging in a lot of anti-Polish pogroms along the way (with some anti-Jewish pogroms, as well) because, well, Polish was the imperial power occupying Ukraine prior to the Russians' taking over. Sometimes you raise up as heroes people who aren't especially pure and virtuous not for their impurity but because of what good they did due. Mickiewicz, the national poet, had a rather bad side to him. At the same time, he did things that were great for Poles.
At the same time, it's hard to dig down through the layers and layers of anti-fashistskii propaganda. I use the Russian word instead of "anti-fascist" because in Russian if you were anti-Russian you were a fascist, and if you were a Nazi you were a fascist, so the term "fashistskii" might mean a RWer who went around killing Jews and loved Hitler or just one who may have hated Hitler and wanted him and the Nazis gone for gone but was still RW enough to think that Stalin and Leninism-Marxism and international socialism were bad, evil things.
Oh, and it pays to note that in both cases they didn't fight Western forces. In fact, the Ukrainian folks may have had German priests with them, but when the Red Army approached and they resumed fighting the "communists" they also made the decision to move to Western Allies' territory with all due haste and not take up arms against the British or the Americans even as the Germans in the areas they transited were busy fighting the Allies. This made for interesting situations, in which the groups were "collaborating" with the Wehrmacht one day and then walking away the next.
It may be the case that the enemy of my enemy isn't necessarily my friend, but the enemy of my enemy is still my enemy's enemy, and my enemy is still my enemy.
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