Late in 1954, the British government convened a secret committee of civil servants to explore the implications of the hydrogen bomb for Britain in a nuclear war. Headed by William Strath, this small and secret cadre of experts thought through the destruction that would follow a thermonuclear attack on the UK. This paper examines the recently declassified “Strath Report” and its ramifications. Drawing on intelligence assessments, technical analysis and economic predictions, its conclusions were profoundly pessimistic. Its portrayal of widespread devastation and the likely collapse of civil society shocked politicians and government officials, and led not only to massive revisions in the UK's plans for war but to official attempts to suppress public discussion of thermonuclear weapons and civil defence.