Answer:
When Roosevelt described United States as the arsenal of democracy he meant that America must act against global tyranny and dictatorship. America must defend democracy and help others who support it as well.
Step-by-step explanation:
World war II started but America did not engage in it. But soon it realized that it needs to interfere otherwise democracy would be at stake.
They could not let The Germans win. United States had to provide ammunition to its allies in order to help them win the war.