In July 1941 the United States, after a decade of worsening economic relations, announced a total embargo against Japan. The embargo had actually begun in 1940 with a so-called moral embargo under which U.S. exports of planes and war material to Japan were barred. In early 1941 Washington squeezed the Tokyo government further by unofficially tightening exports of petroleum. By December 1941, over 90 percent of Japan's oil supply was cut off, as was nearly 70 percent of its overall trade. From contemporary source documents, this is a detailed look at the U.S.-led embargo and how it contributed to Japan's decision to attack Pearl Harbor and declare war on the United States.