The arsenal of democracy is a phrase used to describe how during World War II, the United States became a superpower and helped win the war against Germany and Japan. Think of it like when you play with your toys and you have a lot of different toys to use, it helps you win the game. The United States had a lot of different resources to help them in the war, like factories that made weapons and planes, and a big army of people to fight. It was like having a big box of toys to use to win the war. The US was able to help supply their allies with weapons and equipment too, like giving their friends in England lots of planes to help them fight the Germans. Overall, the arsenal of democracy was an important part of helping win the war by giving the US and their allies the tools they needed to fight back and win.