But this was not an invasion on Ukraine, it was on the Soviets. At that time Ukrainian troops, commanded by ataman Semen Petlura, were fighting the Soviets arm to arm with the Poles, Marshal's Joseph Pilsudski (Polish national leader) idea, was to help to create independent Ukraine and Belarus, allied with Poland. Finally, that idea failed, and - in fact - Poland in Riga behaved towards Ukraine a little bit like US and UK later in Yalta towards Poland, which is shame.
But I can't agree with saying, that Poland concquered any territory from Ukraine, or forced it to cede any teritory. I'd say it was taken from Soviet Russia and not given to the Ukrainians.
As Gefreiter wrote in # 8 "alot depends on how far back we go in considering what "Poland" means". We must remember that it was just after WW1. There was never Ukraine as a state before. In fact, some form of Ukrainian stat existed for a short time after WW1, but was "grinded" during Polish - Soviet fights.
Poland considered itself as a succesor of former Commonwealth of Both Nations (consisting of Polish Crown and Great Lithuanian Duchy) which was destroyed and divided in XVIII century by Russia, Prussia and Austria. Land of Ukraine was a part of that state, within the Crown. Ukraine was supposed to become the third part of the Union, as the "third Nation". But it didn't for many reasons. In my opinion it was one of the biggest mistakes in Polish history, leading to the final fall of the Commonwealth.
So - after almost 200 years of non-existence, when the Poles were rebuilding their state, many of them considered western part of Ukrainian land as Poland (like city of Lvov).
Here is another description.
Poland's defeat of the Soviet Union in that war helps to explain why the Soviets were eager to help Nazi Germany partition it.