In 1939, the first evacuees arrived in Stevenage. Many factories and businesses began producing munitions and supplies for the armed forces. The ESA factory were contracted to supply wings for the de Havilland Mosquito aircraft assembled at Hatfield.

In 1940, Prime Minister Winston Churchill set up the Special Operations Executive (SOE) to run operations in occupied Europe. The new organisation chose Aston House near Stevenage, which was a quiet, secluded country house, as one of the stations to produce explosives for agent to use behind enemy lines. The SOE played a major part in the British War effort, notably the assassination of Reinhard Heydrich, a leading Nazi figure, using devices developed at Aston House and other Hertfordshire stations.