The United States became increasingly isolationist during the 1930s while political turmoil brewed in Europe. Congress passed legislation that would ensure the United States' neutrality, or the position of not taking a side in an armed conflict between other powers. The Neutrality Act of 1937 prohibited the selling of weapons or supplies to allies at war. When German dictator Adolf Hitler invaded Poland in September 1939, the invasion ignited a war against Britain and France—two of the United States' closest allies. President Roosevelt wanted to help these allies without drawing the United States into the war. He persuaded Congress to amend the neutrality law. The Neutrality Act of 1939 offered a compromise known as "cash-and-carry." The United States would provide airplanes, tanks, fuel, and food to its allies, provided they paid in cash and transported the materials on their own ships. Through this policy, the United States hoped to avoid debt and being pulled into a foreign conflict, which had happened when the country became embroiled in World War I. As the cash-and-carry program went into effect, the U.S. economy got a boost from increased production of the goods and supplies European allies purchased.In 1940 after the fall of France and the Nazi invasion of most of northern Europe, it became clear cash-and-carry was insufficient help for the country's cash-strapped allies. Britain in particular needed airplanes and battleships but could not pay for them in cash. Roosevelt was determined to help Britain as much as possible without committing the United States to entering the conflict. He therefore pressed Congress to pass the Lend-Lease Act in 1940. The Lend-Lease Act of 1940 was a law giving the president the power to lend equipment and supplies to vital U.S. allies. The terms of repayment were also to be determined by the president. Lend-lease was controversial. The bill inspired the slogan "send guns, not sons" among its supporters. Opponents feared lend-lease was "a blank check bill" that might drag the United States first into debt and then into war. Congress approved the Lend-Lease Act in 1941. Over the course of the war the United States sent 50 billion dollars in Lend-Lease aid and equipment to allies, including Britain, France, China, and the Soviet Union. Not unexpectedly, the Axis powers perceived lend-lease as the unofficial end of American neutrality. Soon after its passage, German submarines, called U-boats, began sinking unarmed American merchant ships in the Atlantic.