This article traces the origins, development, and demise of the Third-Line Defense project in the People's Republic of China (PRC) from 1964 to 1966. Responding to the U.S. escalation of the Vietnam War, Chinese leaders decided to transfer strategic military and civilian assets from the vulnerable coastal and border provinces to the country's interior. Following the dispatch of U.S. Marines to Vietnam in March 1965, the PRC proceeded with the construction of provincial Third-Line Defense projects. In the end, the Third-Line Defense project fell victim to Mao Zedong's ideological radicalization in the lead-up to the Cultural Revolution. The article uses documentary evidence from Chinese provincial archives as well as published collections of Chinese documents.