Second Sino-Japanese War
In 1932, Japan had invaded Manchuria and established the puppet state of Manchukuo. In 1937, Japanese military forces attacked the Marco Polo bridge in near Beijing. On July 7, 1937 Japanese troops stationed in North China, under terms of the Boxer Protocol of 1900, clashed with elements of the Kuomintang army near the Marco Polo Bridge, ten miles west of Beijing. Three days after the battle, Chiang Kai-Shek declared Total War on Japanese forces in China. An all-out Japanese invasion of 400,000 soldiers followed. By the end of the month, Beijing was occupied and the following month Shanghai (See Battle of Shanghai), Nanjing, and Shanxi were attacked. Japanese military forces eventually gained control of all three cities, though were met with fierce Chinese resistance.
In the U.S., the The Neutrality Act required the President to implement its provisions when a state of war existed, but Roosevelt did not invoke the act. FDR permitted American munitions to be shipped via British ships destined for Hong Kong. Time magazine speculated, "though in the long run Japan might need U. S. supplies more than China, the immediate effect of an embargo would be to the advantage of Japan, whose ships could still presumably conduct a "cash and carry" trade with the U. S."
Most of the Japanese attacks were launched against Nationalist-held territory, not Communist. Thus Chiang's policy of giving up territory and not launching counter-attacks was interpreted as lack of will to fight Japan.  considering the fact that the communists held zero territory at the time, this was not a surprise.
At the outbreak of the war, the Republic of China had little to no mechanized forces, inferior equipment, and poorly trained soldiers. Due to this, Japanese forces quickly swept through China and captured most of the developed portions of the country.
Though Japan had no problem conquering most of China, guerrilla fighting continued throughout the conquered areas.
- Prior to 1939, China and Nazi Germany had seen close economic and military ties. After the outbreak of war in Europe, Germany was generally reluctant to help Chinese forces.
- The Soviet Union provided the largest amount of foreign aid to China during the war, some $250 million worth of supplies. 
- The United States aided in supplying materiel to the Kuomintanguntil the bombing of Pearl Harbor in December 1941. Soon after going to war with Japan (see Pacific front of World War II), the US developed a volunteer fighter wing known as the Flying Tigers, which saw great success over Chinese skies (300 kills compared to 12 losses).
- Manslaughter in Shanghai, Time magazine, Aug. 30, 1937.
- Anne W. Carroll, Who Lost China, 1996. Retrieved from www.ewtn com/library/ August 16, 2007.