"The enemy of my enemy is my friend." So China and the United States were friendly in the early days of World War II, even before Japan attacked Pearl Harbor.
A covert military operation brought American planes and pilots to Southeast Asia to support the Chinese in their fight with Japan: The Flying Tigers.
The story of the group's creation and activities is told in Samuel Kleiner's book Flying Tigers: The Untold Story of the American Pilots Who Waged a Secret War Against Japan.
It's a highly unusual story, this group of volunteers fighting under a foreign flag. The author visits to unfold the tale.