Moral Embargo

On July 1st, 1938, as a result of the wide-spread bombing of noncombatants in China by the Japanese, which not only caused outrage in the United States, but also affected American inhabitants of China, the US Department of State notified manufacturers and exporters of aircraft that the US Government was strongly against the sale of such equipment to countries whose armed forces were using airplanes for attacks on civilians.

In 1939, it was extended to materials essential to aeroplane manufacture and operation. 

Overall, the moral embargo basically halted the exportation of arms to Japan.