http://www.np.reddit.com/r/paradoxplaza/comments/2115ed/any_historical_examples_of_the_dishonored/cg8t7e2

Rule 5: The French and British did in fact declare war on Nazi Germany when Germany invaded Poland. While one could find fault with the initial Allied conduct of the war, the fact of the matter was that Poland was doomed. Cut off from all support by the Baltic Sea and the bulk of Germany and facing a two front invasion by the two premier military powers in Europe, there was little that the Allies could have done.

Furthermore, the Franco-British forces were not really suited for offensive actions into Germany. The French in particular had an army optimized for defensive warfare (Maginot Line anybody), and France was wracked by deep internal divisions. Britain had a small standing army, and its strength was rooted in its navy.

The atrocities committed by Nazi Germany were most certainly not the fault of the Allies being unable to defend Poland. Nazi Germany was already well on the road that led to the Holocaust, and the Hunger Plan that attempted to exterminate the Slavic population of Eastern Europe came from Hitler's ideological obsessions, not Allied military failures.

As an aside, the post isn't even relevant to the thread its in. The thread is about historical examples of a Casus Belli in Europa Universalis 4 called "Dishonored Call", when you call an ally to war and they refuse. Poland most certainly did not have the French and British refuse their initial call to arms, and then declare war on the Allies in revenge.