It was not clear why Mr. Brown, who served over 24 years in the U.S. Air Force and left active duty in 1996 with the rank of major, agreed to help the Chinese military. Prosecutors said, however, that he appeared eager to begin after negotiating contract terms around August 2023. According to the Justice Department, he told a co-conspirator, “Now … I have the chance to fly and instruct fighter pilots again!”