Before the U.S. officially entered the war in December 1941, it unofficially (but very obviously) supported the Allied countries in their fight against Nazi Germany's expansion. The most powerful way that the U.S. did this was through the Lend-Lease Act, which allowed the U.S. to "lend" military equipment to countries if doing so helped defend America.

This speech, given at the White House Correspondents' Dinner (before it turned into a comedy show, we guess) on March 15, 1941, presented FDR's arguments in favor of the Lend-Lease program. He described the Nazi agenda:

They openly seek the destruction of all elective systems of government on every continent—including our own; they seek to establish systems of government based on the regimentation of all human beings by a handful of individual rulers who have seized power by force. [...]

These modern tyrants find it necessary to—their plans to eliminate all democracies—eliminate them one by one. The Nations of Europe, and indeed we ourselves, did not appreciate that purpose. We do now. The process of the elimination of the European Nations proceeded according to plan through 1939 and well into 1940, until the schedule was shot to pieces by the unbeatable defenders of Britain.

Props to Roosevelt for giving us a pretty succinct description of why the Allies were fighting against the Nazis.

Several times, Roosevelt referenced how other nations saw America as a seriously divided country, whose infighting and internal discord would prevent it from ever being a threat to others. Not so, said Roosevelt: "We do not have and never will have the false unity of a people browbeaten by threats, misled by propaganda. Ours is a unity that is possible only among free men and women who recognize the truth and face reality with intelligence and courage.

He referred to Lend-Lease as the "aid-to-democracies bill" and he was pretty honest about what this program would do:

Yes, you will feel the impact of this gigantic effort in your daily lives. You will feel it in a way that will cause, to you, many inconveniences. You will have to be content with lower profits, lower profits from business because obviously your taxes will be higher. You will have to work longer at your bench, or your plow, or your machine, or your desk. Let me make it clear that the Nation is calling for the sacrifice of some privileges, not for the sacrifice of fundamental rights. And most of us will do it willingly. That kind of sacrifice is for the common national protection and welfare; for our defense against the most ruthless brutality in all history; for the ultimate victory of a way of life now so violently menaced. 

Here you see rhetoric that you'll find in the speeches of a lot of World War II leaders, about how everyone will need to sacrifice in order to defeat the Nazi threat. Roosevelt was calling on the American people to chip in, especially to help those in Britain, who were "bravely shielding the great flame of democracy from the blackout of barbarism."

He spent the remainder of the speech outlining what the U.S. would provide to the Allies, and why Americans should support the effort. There are some great lines in there about the difference between loyalty and obedience, but we won't spoil it.

Oh, and he also mentioned how "Britain is blessed with a brilliant and great leader in Winston Churchill." 

(Source)