The American Volunteer Groups were volunteer air units organized by the United States government to aid the Nationalist government of China against Japan in the Second Sino-Japanese War. The only unit to actually see combat was the 1st AVG, popularly known as the Flying Tigers.
They ceased operations officially after the Japanese victory at Midway in 1943, at which, the US fleet was largely destroyed and the US was forced to the negotiations table. The US government promised to stop aid to the Chinese, and ordered the groups to return to the US. However, many secretly deserted and continued fighting in China until the war ended with a Japanese victory in 1945.