For centuries few bothered to actually check Poland out. Until about 1945 there was not a serious book on Poland's history, culture, politics outside Western Europe/USA that had not been largely a translation of German sources.
Yes, this is the most accurate. It started with Prussia, Germany and the partitions. There were then mass immigrations of both Germans and Poles to the U.S. where it continued, and then carried over to the media up until the 1970s.
en.wikipedia.org/wiki/Anti-Polish_sentiment
(I've read this elsewhere but cannot be bothered to find the articles right now).
Uhm, and I'm pretty sure America is not the only place that has harbored ethnic slurs.