Poland. The Third Reich, let by its Fuhrer, Adolf Hitler, was displeased with stripped land as a consequence of the Treaty of Versailles due to Imperial Germany losing WWI. He entered an alliance with the Soviet Union, led by Joseph Stalin, where they both invaded Poland and roughly split it by half.
The British Empire and France both simultaneously declared war on Germany, as they had guaranteed Polish independence, but DID NOT declare war on the Soviets.
Poland was quickly split by the powers and a phony war ensued while Hitler pursued other plans.