The Lend-Lease Act signaled to the world that the United States was getting involved in WWII.

If your knowledge of the most terrible war in human history only comes from Hollywood films and video games (and let's face it, that's how a lot of us do), it's easy to think WWII came out of nowhere. Like it was some sort of lightning strike of insanity, a sudden eruption from the dark.

And it's true that the actual fighting came fast and hard and completely shattered millions of lives, sometimes in a matter of days. Operation Barbarossa, for example, Germany's invasion of the Soviet Union, was the largest military offensive in human history, and involved millions of men.

But the rise of Hitler and the buildup of the Wehrmacht (German army) took years, and the war had been officially going on for two whole years before Pearl Harbor and America's declaration of war on Japan, Italy, and Germany (about four years if you count Japan's invasion of China).

World War I, at the time known as "The Great War," ended in 1918. By the end of it, over 100,000 American troops had died. As terrible a price as that was, the United States actually entered the war very late, and got off easy, (very) relatively speaking. Tens of millions of soldiers and civilians were killed and wounded in Europe. You can understand, then, why many genuinely hoped it was "the war to end all wars."

Spoiler: it wasn't.

In 1929, the New York stock market crashed, marking the beginning of the Great Depression, a worldwide economic catastrophe that left very few nations untouched. The hapless Republican President Herbert Hoover, by most accounts, made the Depression worse with his policies, which were inspired by his philosophy that government should mostly get out of the good people's way. Probably the most infamous misstep of his was signing the Hawley-Smoot Tariff (yes, that is really what it was called), which levied a fee on imports to encourage domestic spending, but which caused retaliation from trading partners.

Hoover came into office at the very end of the "roaring '20s," and foolishly said something to the effect that poverty in America was almost at an end. A year later, the worst economic crisis in American history got off to a roaring start, making him look the ultimate fool.

Needless to say, Hoover wasn't all that popular by the time the 1932 Presidential election came around. With unemployment over 20%, shantytowns forming around big cities (dubbed Hoovervilles by Democrats), and Hoover's administration raising taxes in a futile attempt to balance the budget, Franklin Delano Roosevelt crushed him by promising the American people a "New Deal."

On the other side of the world, in Germany, a strange, half-mustachioed man named Adolf Hitler was coming to power. Maybe you've heard of him.

His radical "national socialism" philosophy (though it's a bit of a stretch to use that word) and anti-Jewish bigotry found wide-ranging support from the German people, both because of the Depression, which had hit Germany especially hard, and because of the humiliation of Germany's defeat in WWI (and enormous resentment of the war reparations they were forced to pay). In 1933 he was appointed Chancellor, and in 1934 appointed himself Führer, or "leader" of Germany.

Somewhat understandably, many people were keen to see Hitler, his Italian buddy Mussolini, and the other assorted fascist copycats scattered across Europe as mostly harmless. To such optimists, another "Great War" was unthinkable. In the early 1930s, American popular opinion in particular was decidedly non-interventionist. To a lot of people, Europe seemed a world away.

That remained the majority opinion in America even after Hitler started making pretty serious moves (for instance, marching into Czechoslovakia…). Americans didn't want to get involved in another European war, whether for moral, economic, or diplomatic reasons. Maybe it's simplest to say it was just the zeitgeist (German for "spirit of the times," but as we've noted elsewhere, could be literally translated as "time ghost." We'll let you decide which translation to use).

President Roosevelt didn't agree with that part of the zeitgeist, and he spent much of the '30s trying to get Congress to let him direct military aid to Britain, France, China, and the U.S.S.R., while using some of his public addresses and "fireside chats" to shift popular opinion.

But even after the Nazis reneged on their WWI reparations, started full rearmament, and annexed both the Sudetenland and Czechoslovakia, there was still widespread political opposition to even appearing to take sides in the burgeoning conflict.

Then the Nazis invaded Poland in 1939. And France in 1940. And then they started bombing Britain. Headlines about the brutalities in Europe, newsreels, and anti-German propaganda started to turn American popular opinion, giving Roosevelt the legislative support he needed to pass the Lend-Lease Act.