An interesting theory as to why Hitler singled out the Poles, compared to the other Slavs, like Czechs or Slovaks....the theory is that since many Poles had a lot of Germanic blood in them as well, they were perceived as more of threat, so according to the Nazis, Poles and Germans simply could not co-exist. Note, the Nazis never employed Poles as guards at the death camps, although they did use other Slavs, such as Lithuanians and Ukrainians.......the Germans figured that the Poles would immediately fire on them if given rifles, they were probably right.
At the same time they associated themselves with southern slavs like Bulgarians and Croats
It was pure convenience -- Poland was in the way, just as Poland was in the way of Russia
The problem with Poland is that it's not Germanic enough to fit in with the West, and not "Slavonic" enough to fit in with the east.
for that matter, the entire section of countries from Finland to Croatia fall into that basket, hence the proper need for Międzymorze.