The invasion of Poland was not difficult for the German army. However, it was not as easy as the German occupations of Austria and Czechoslovakia.
The Germans did not actually have to invade either Austria or Czechoslovakia in any serious way. In the case of Austria, there was no military component. In Austria, there was an internal coup d’etat in March of 1938. This was carried out by the Austrian Nazi Party. The party carried out the coup to prevent a scheduled referendum on whether to unite with Germany. The Austrian Nazi Party then ceded power to Germany and German troops entered Austria by invitation. The case of Czechoslovakia was not much different. The Czech president “voluntarily” allowed Germany to enter the country since the Germans had threatened to bomb Prague. There was essentially no military resistance when the Germans entered the country.
By contrast, there was actual resistance to Germany’s invasion of Poland. Poland did not consent in any way to be taken over by Germany. However, there was not much difficulty for the Germans. The Poles had borders that were very hard to defend geographically and they had a very inferior military. Therefore, the invasion of Poland was completed in just over one month.