Starting in December 1937, events such as the Japanese attack on USS
Panay, the Allison incident, and the Nanking Massacre swung Western
public opinion sharply against Japan. Fearing Japanese expansion,the
United States, United Kingdom, and France assisted China with its
loans for war supply contracts.
In 1940, Japan invaded French Indochina, attempting to stymie the flow
of supplies reaching China. The United States halted shipments of
airplanes, parts, machine tools, and aviation gasoline to Japan, which
the latter perceived as an unfriendly act. The United States did not stop
oil exports, however, partly because of the prevailing sentiment in
Washington: given Japanese dependence on American oil, such an
action was likely to be considered an extreme provocation.