Japanese invasion of French Indochina
The United States strongly supported China starting in 1937 and warned Japan to get out. However, the United States continued to support Japan with petroleum and scrap metal exports until the Japanese invasion of French Indochina which forced the U.S. to impose the scrap metal and oil embargo against Japan (and freezing of Japanese assets) in the summer of 1941. As the Soviets prepared for war against Nazi Germany in June 1941, and all new Soviet combat aircraft now destined to that war-front, Chiang Kai-shek sought American support through the Lend-Lease Act that was promised in March 1941.