Final answer:
The Free City demanded by Hitler to be returned to Germany was the Free City of Danzig, which led to escalated tensions contributing to the start of World War II.
Step-by-step explanation:
The "Free City" that Adolf Hitler demanded to be returned to Germany from 1920-1939 was the Free City of Danzig. This city, now known as Gdańsk, was a semi-autonomous city-state that was established under the Treaty of Versailles to provide Poland with access to the sea while keeping it separate from German territory due to its predominantly German population. Hitler's demand was part of his greater foreign policy to unite all German-speaking peoples and expand territory for the perceived need for Lebensraum (living space). This policy led to the annexation of Austria and the Sudetenland, followed by the occupation of the rest of Czechoslovakia, contrary to the appeasement attempts exhibited at the Munich Pact. Eventually, Hitler's aggressive expansionism and the invasion of Poland in September 1939 triggered the start of World War II.