The hemispheric defense zone was President Franklin D. Roosevelt's response to the Germans sinking of British and American ships transporting supplies and armaments to England. Although the United States was a neutral country, FDR created the hemispheric defense zone to aid the British and expand the area where they were allowed to defend if the Axis powers were a threat.
The 1823 Monroe Doctrine deemed the United States neutral unless invaded or threatened by any Eastern Hemisphere countries. Though not active participants in the pre-WWII battles in Europe, America passively participated by contributing arms and even training to the British military. When the Brits ran out of funds to pay for this aid, the U.S. established the Lend-Lease Act, which stated that they may lend and lease to countries essential to the defense of the United States. When this aid commenced, Germans began to sink the ships carrying the supplies. In an effort to keep out of warfare, FDR expanded the Atlantic boundary of the Western Hemisphere and placed the U.S. Navy there for protection and to monitor the location of German ships. The Navy sent this information to the British, eventually leading Hitler to abandon his plan to invade England and turn his focus onto the Soviet Union.