The problem of getting American arms and supplies to Britain remained. German submarines in the Atlantic sank hundreds of thousands of tons of cargo each month. The British Navy lacked the ships to stop them.
Because the United States was still officially neutral, Roosevelt could not order the navy to protect British cargo ships. Instead, he developed the idea of a hemispheric defense zone, declaring that the entire western half of the Atlantic was part of the Western Hemisphere and therefore neutral. He then ordered the U.S. Navy to patrol the western Atlantic and reveal the location of German submarines to the British.