After Hitler annexed Austria in 1938, the Gestapo began silencing critics. Many were shipped to concentration camps; those deemed most dangerous to the Reich were executed. Yet a few slipped through the Gestapo's net and organized resistance cells. One group, codenamed CASSIA, became America's most effective spy ring in Austria during World War II. This first full-length account of CASSIA describes its contributions to the Allied war effort—including reports on the V-2 missile, Nazi death camps and advanced combat aircraft and tanks—before a catastrophic intelligence failure sent key members to the guillotine, firing squad or gas chamber.