With the split of Austro-Hungary, Austria was for first time in 600 years demographically South German/Austrian (of course with some minorities). Many were actually keen to join the German state. They held a referendum, but worried that they wouldn't like to join Nazi Germany, the Nazis marched into the country to make sure they got result they wanted.
While the Polish did fight bravely, their war machine was not up to the task and they were isolated and could not receive necessary support from the Allies.
After France fell, Northern France was under Nazi occupation, while the south became the puppet state of Vichy France. The colonies fought on with the allies however. Many battles in Africa were fought to enable the invasion through the Mediterranean into Sicily.
The Maginot Line in France could stop the Germans, but German foolhardiness and luck had German Armor divisions penetrate deep from the Low Countries (Netherlands, Belgium and Luxembourg) to allow encirclements of massive amounts of soldiers. The Allies were split up and lost organization and had to pull back from Mainland Europe.
Didn't mean to be a history lesson, just some background on the circumstances that led up to the Allies getting pushed out of Mainland Europe.