The story
Introduced to wide usage (although probably not first used) by U.S. President Franklin Delano Roosevelt in December, 1940.
Etymology from Wiktionary, CC BY-SA 4.0.
arsenal of democracy
Today it means: The United States of America, as supplier of extensive materiel support to U.S. allies during World War II; or post-war U.S. as the manufacturer and possessor of the greatest quantity of advanced military weaponry in the world, in the service of defending governments favouring the United States, mostly democracies. Full definition →