The National Environmental Policy Act (NEPA) is our environmental Magna Carta. Pressured by the largest citizen movement of its time, NEPA was signed into law on January 1, 1970 by then President Nixon.
For the first time, the public was given a fundamental right to participate "to the fullest extent possible" in planning federal projects with the potential to harm our public lands, wildlife habitat, climate, as well as the air we breath and the water we drink.
NEPA put federal agencies on notice mandating "public scrutiny" and "accurate scientific analysis" of their actions to achieve broad national aims of protecting, restoring and enhancing the human environment.
These regulations interpret the law and are enforceable in federal court.The courts have found that agencies must take a "hard look" at all impacts, including "cumulative impacts (that) can result from individually minor but collectively significant actions." Agencies must also present reasonable alternatives to planned actions, including doing nothing. Environmental Impact Statements (EIS) and Environmental Assessments (EA) are required for major actions. If the agency determines the environmental consequences are minor, their decision must documented in a Categorical Exclusion (CE).
Here are parts of NEPA from The Code of Federal Regulations (CFR), given here for reference: