March 20, 1921. The Upper Silesian Plebiscite

"Vote for Poland and you will be free", a pro Polish campaign poster.  Interestingly, while the vote would go on largely ethnic lines, this poster was in Polish and German.

A plebiscite was held in Upper Silesia to determine its national fate. The result apportioned the territory between Poland, Germany and Czechoslovakia. 

This would, of course, help set the game board for World War Two, as did the Treaty of Riga from the day prior.  Germany wasn't content with the results, and in actuality Poland really wasn't either.  When Germany dismembered Czechoslovakia in the following decade, Poland took a piece, although I think of lower Silesia and other border areas, before it soon faced Germany's territorial expansion itself.  Czechoslovakia took them back in October 1939 and then the border returned to its 1920 line following World War Two.

Also following World War Two almost all of Upper Silesia was placed in Poland.  Interestingly, unlike Lower Silesia, not all of its ethnic German population was expelled as some of it was bilingual and as the Germans in Upper Silesia were Catholic, and somewhat intermixed with the Polish population, some were allowed to remain.  The region currently has a small autonomy movement.