Manhattan Project kept secret by need-to-know compartmentalization

By 1943 the Manhattan Project employed well over 100,000 people across sites including Los Alamos, Oak Ridge and Hanford, running uranium enrichment plants, plutonium production reactors, and weapons design labs simultaneously. Vetting every worker to the highest security clearance and trusting each one individually with full knowledge of the project's purpose would have been impossibly slow at that scale, and the more people who understood the whole picture, the more ways existed for a single leak, a careless letter, a captured spy, an indiscreet conversation, to hand an adversary everything.

General Leslie Groves, who oversaw the project's security, pushed for tight control over what any individual was allowed to know, extending beyond written material to oral conversation: workers were urged to think before speaking even informally, and Los Alamos director J. Robert Oppenheimer, while valuing free internal scientific communication, conceded this required real limits on who could access what information and discuss it with whom.