The cause of World War 2 is always said to be Germany's invasion of Poland, but why don't they add a most...?

The only territory in Poland that was governed by Germany before World War I was East Prussia, and even that was not 'German' or Prussian before the partition of Poland between Russia, Austria and Prussia in the 1790s. So that part of Poland was 'German' for only 120-30 years, and that only by conquest of the Poles.