At the start of the Middle Ages (the 400s), the Germanics like Goths, Vandals etc. were moving out of southern Sweden and Denmark and moving into what is now central Europe and then western Europe. They pushed the Celts and the Romans (Italics). They called the Celts and Romans as Wlachs (foreigners, note -- the Greeks called non-Greeks as barbarians), hence the common terms Welsh, Walloon and Wallachia
They stayed in what is now poland for cneturies until moving out in the 600s
Then the Western Slavs came in (Polanians, Vistuans, Kashubians, Czechs, Moravians etc.) and the Southern slavs moved to the south, pushing the Thracians and Greeks.
Then, in the 9th century the Magyar hordes came and permanently separated the southern slavs from the western slavs by the Hungarian-Romanian band
The Germans were busy setting up the Germanic kingdoms of France, Visigoth Spain, Lombard Italy etc. and the Slavs basically walked in.
Then, in the 11th century the Germans turned eastwards and wanted a piece of the action
To the Slavs the Germanics were also bringers of high culture -- but it was difficult -- how to retain their language and culture while taking the aspects of high civilisation.
The Southern and Eastern Slavs fell under the influence of the Greeks and only St. Cyril and Methodius' created of Galgothic andthen Cyrillic stopped that, but the Eastern and Southern slavs were under heavy Greek influence until well into the 1800s
Among the WEstern Slavs, the Czechs fell into the German orbit and became a part of the holy Roman Empire.
the Poles wisely stayed out (well, not wisely, they were slower in progressing than the Czechs so they saw what was happening to the Czechs)
this started the millenia of German-Slav "relationship". This was exacerbated in the 1200s when Konrad the prince of Mazowia in Poland called in the Teutonic knights to fight the Prussians (a Baltic people) for him. The knights came and stayed and set up their own kingdom after commiting genocide on the Prussians. This was Prussia
The knights were a threat to Lithuania and to Poland and they were defeated by a joint Polish-lithuanian-ruthenian (future Belarussian and Ukraianin) and Tartar army at Grunwald in 1410.
as an aside, 500 years later when the Germans defeated the Russians some 100 km from this site at the start of WWI, they called it the battle of Tannenburg (German for Grunwald) part II -- revenge :)
BUT let's not simplify this as a simple German-pole fight. There was common blood shared and for a long time people were happy being of German origin in Poland.
Then came the partitions of poland 1789-1795.
In the Prussian part there was not a strong emphasis on German-ness until the formation of the German Empire. Then, since 'Germany' was a new state, a focus to make it a purely German state was started -- especially under Bismarck. Bismarck also in his Kulturkampf tried to destroy the Catholic Church in Germany -- he only succeeded in making German families in Poland get disgusted and become heavily Polonized (see the Wedel family)
from the late 1800s we see this new idea of a perpetual German-Polish struggle arising
Bismarck inflated this when he blamed the Poles for their partitions comparing this to their defeating the Prussians
hitle took this up and expanded it -- between Germany and Poland there could be only one, no way out.
The Kulturkampf is the reason my ancestors came to America.