The Great Arsenal of Democracy is one of the most famous of 30 fireside chats broadcast on the radio by United States President Franklin Delano Roosevelt. It was read on December 29, 1940, at a time when Nazi Germany had conquered much of Europe and threatened Britain. Germany was allied with Italy and Japan, and was collaborating with the Soviet Union. Franklin Roosevelt had referred to Detroit as the "Arsenal of Democracy," when the auto industry geared up to produce weaponry during World War II.

The speech was "a call to arm and support" both Europe and to a lesser extent Asia's powers in their respective struggles against the fascist regimes. At the time it was broadcast Roosevelt assumed the Axis powers of Germany, Italy and Japan were collaborating closely in their successful wars in Europe and Asia. The term "arsenal of democracy" was contributed by Harry Hopkins, Roosevelt's top advisor.

In terms of history, the speech has been interpreted as the "next step" in a several stage process in "awakening" a peaceful country that had been isolationist for the preceding two decades. While the United States Navy seemed strong and was widely perceived to guarantee the Western Hemisphere safe from invasion, the United States Army contained barely 250,000 soldiers.