What a nazi crap here! Probably in Germany you could be judged for claiming such things!
Thinking this way would lead us to new war.
What is making me feel better is that you do not live in Germany, only in US and you're far away from Europe.
To be correct: Prussians were separated ethnic group, they had nothing to do with Slavians. Polish prince Konrad Mazowiecki invited German Teutonic Order to those areas to fight Prussians - they were pagans, so such Order was keen to bring Christianity there (with sword - strange idea, but that's how it worked then). This Order grew and finally started to pose a threat to Poland (later to Commonwealth, when Poland and Lithuania formed an union). Order lost wars with Commonwealth in XV century and ceased to be a threat. Then it was secularized into Prussia.
In 1945 lands of Prussia was taken by Russians, slices were given to Poland and Lithuania. Today there is no Germans there as they were expelled right after the war.
About other parts of Germany which are now in Polish borders - first of all we, Poles would like to keep our pre-war borders, we lost many historical cities in the east and literally millions of Poles were expelled by Russians from there and forced to move into new lands.
Last thing - about historically German lands - what is ironic it that those lands which we gained in 1945 were Polish long times ago. In the time when Poland was formed - in X century Wrocław (Breslau) and almost whole territory that is now Polish was then Polish. For centuries Poland shifted east and when in 1945 Russians, Americans and English draw a map of Europe once again, Poland was shaped in almost the same way as it looked like over 1000 years ago, when it emerged.
So, thinking in your way, we get what was ours

(at least that was the official, communistic propaganda after WW2).