Brief Summary
The Set-Up
Ron Swanson believed that the less he knew about other people's issues, the happier things are.
But as much as we love Mr. Swanson, U.S. citizens couldn't just ignore what was happening in Europe (*cough* WWII *cough*). So FDR found a compromise: the best way to help wasn't to send troops overseas, but to mobilize American industry to produce the weapons and supplies the Allies needed to win.
The Text
FDR was a fan of Fireside Chats. And we're not talking literally: Franky D. wasn't eating s'mores and telling ghost stories, he was talking to the people of America via that newfangled device, the radio.
So, when he gave his "Great Arsenal of Democracy" speech over the radio, he knew lots of people would tune in. (In general, people loved FDR.)
FDR spent the entire first half talking about all the bad things the Axis were doing to U.S. allies, not because he enjoys spreading bad news in dulcet tones, but because he had to convince the American people to care about the war. If it was bad for U.S. allies, it was bad for America, and he'd keep saying it 'til everyone believed it.
As far as he was concerned, the American people had two choices: they could wait for the Axis to attack, or they could be proactive and start preparing to defend themselves. During the second half of his speech, FDR advocated for being proactive—producing weapons, sending supplies overseas, and rebuilding military reserves to bolster U.S. defenses on the homefront.
And there wasn't any time to waste. Hitler wasn't going to play fair.
TL;DR
"Of course the war is happening across the world, Harry and all Americans," said Franklin Dumbledore Roosevelt, "but why on earth should that mean that it is not a real threat and we shouldn't make lots of guns and tanks to protect everyone?"
Sounds about right—we think J.K. Rowling would appreciate our rewrite.