Polish and Italian gangs were the most numerous in Chicago (circa 1910). Polish gangs were located on the Northwest Side and often fought rival Polish punks on the south side as well as in the Bucktown area and along the Chicago River. These gangs also engaged in territorial skirmishes with Italian gangs of the "Little Sicily" neighborhood. Then came the Moran (who was half Polish) skirmishes with Capone which pretty much ended after the St. Valentines Day massacre initiated by Capone.
One interesting character was Jake "Greasy Thumb" Guzik who worked for Capone. en.wikipedia.org/wiki/Jake_Guzik
Another Polish American gangster was Earl "Hymie" Weiss (who despite the middle sobriquet was Polish Catholic.
Earl "Hymie" Weiss was a Polish-American gangster who became a leader of the Prohibition-era North Side Gang and a bitter rival of Al Capone.