Caldwell is perhaps best known as one of the principal architects of the National Environmental Policy Act (NEPA), the first act of its kind in the world, signed into law on January 1, 1970. In drafting "A National Policy on the Environment" in 1968 as consultant to Senator Henry Jackson, the head of the powerful Senate Interior and Insular Affairs Committee, Caldwell realized that more was needed than a mere a policy statement: an “action-forcing mechanism” would be necessary to secure federal agency compliance with the Act’s requirements. The origin of the requirement for preparation of an environmental impact statement (EIS) has been attributed to Caldwell, whose testimony at the Senate hearing in April 1969 laid the groundwork for inclusion of provisions requiring an evaluation of the effects of all major federal projects significantly affecting the quality of the human environment. In these “detailed statements,” as they were termed in the Act, all reasonably foreseeable social, economic, and environmental effects of a proposed action and any possible alternatives to it must be identified and assessed before any federal action takes place.