The AVG (American Volunteer Group) was not affiliated with the US Military. They were Volunteers, basically Mercenaries, as the name implies. The US was not involved in combat missions against the Japanese before Pearl Harbor.
The US was not "officially" fighting in China until after we entered the war. Here in Shanghai, the AVG is uniformly appreciated and practically worshipped. The AVG was actually an OSS (precursor to the CIA) operation, which was designed to slow the Japanese down until the US could enter the war. After Pearl Harbor, it quickly morphed into an official US Army Air Corps unit, which is where my Dad was stationed. Many of the pilots stayed - but many were just flat out mercenaries hired by the OSS - and weren't interested in the drastic wage cuts that absorption into the Army would entail.
I have the autobiography of one AVG member ("God is My Co-pilot" by Robert Lee Scott)
Scott was an active-duty military pilot. He and his friends were approached by an officer of the US military the the US would release them from active duty if they chose to volunteer to go to China and fight, and would be allowed back into the US military at a later point in time with no loss of seniority
Stripped of the "plausible deniability", these were US military who volunteered to fight the Japanese prior to the official start of hostilities with the full blessing and support of the US military