KORNTAL, Germany—Research by a born-again Christian anthropologist working alone from a cramped desk in this German suburb thrust China and the West into one of their biggest clashes over human rights in decades.

Doggedly hunting down data in obscure corners of the Chinese internet, Adrian Zenz revealed a security buildup in China’s remote Xinjiang region and illuminated the mass detention and policing of Turkic Muslims that followed. His research showed how China spent billions of dollars building internment camps and high-tech...