A few years later in 1937 a wider war in China did indeed begin. Japan claimed that its troops had been fired at by Chinese soldiers at Marco Polo Bridge, and soon after its army invaded China from Manchuria. The Japanese army dominated the early period of this war as they advanced further into China. One by one China's cities such as Nanking and Shanghai were occupied.
Notwithstanding its isolationist foreign policy, the United States did establish a Japanese embargo in 1940 as the war in China continued. These economic sanctions froze American oil exports to Japan, as well as exports of steel and iron ore. Even though the embargo was incomplete, since Japan still obtained a small quantity of oil from the United States, it began to have a noticeable impact by 1941. As Japan's oil supplies became increasingly depleted, it was clear that it could not continue the war in China much further as aircraft and tanks required fuel.