Or, at least, it worked. The Trump administration has indicated—through its proposed budget and through its choice of appointees—that it cannot abide the status quo. Its proposed budget cuts billions from the agency’s budget, and it has begun the process of rescinding years of Obama-era regulatory work.

Trump could be the most hostile president ever to sit over the agency. His only rival is Ronald Reagan, who did not enjoy the benefit of a Republican Congress. Suffice it to say that this scares a lot of Americans. Many of them have looked anew at the environmental policy machine running in the background of the government and asked, essentially: Wait, that old thing? How does that work?

This is a brief guide to how it works.

How do we protect the environment in the United States?

We mostly do it with statutes and regulations. A statute is a law passed by Congress, while a regulation is a law promulgated by a federal agency.

The process works like this: Congress passes a law with a general goal in mind—say, cleaner air around the country. This statute formally empowers the EPA, an independent agency of the federal government, to issue regulations about what companies must do to help bring about that cleaner air. Congress also gives money to the EPA to enforce those rules. Some of that money is supposed to go to states, who will enforce some of the regulations themselves.

What are the most important laws governing the EPA?

There are two crucial ones, passed by Congress within a two-year span of Richard Nixon’s presidency:

The Clean Air Act of 1970 tells the EPA to set standards for what kinds of toxic air pollutants can be released into the “ambient air,” either from factories or cars and trucks.

The Clean Water Act of 1972 tells the EPA to set standards for what pollutants can be released into lakes, streams, and rivers, and it forces polluters to get permits to do so.

When these statutes were passed, they were popular, bipartisan bills. Nixon signed the Clean Air Act in a well-publicized ceremony.* “I think that 1970 will be known as the year of the beginning, in which we really began to move on the problems of clean air and clean water and open spaces for the future generations of America,” he told reporters.

There are two more laws that don’t directly affect the EPA as much, but which come from the same period and expanded the government’s environmental power:

The National Environmental Policy Act of 1970 (NEPA) requires the federal government to conduct a lengthy environmental-impact study every time it wants to build, approve, or renovate something.

The Endangered Species Act of 1973 lets NOAA and the Fish and Wildlife Service protect species at risk for extinction, granting the U.S. government huge powers in the process. (This is partly because it was drafted by environmentalists and quickly signed by Nixon, who sought to give the press a Christmastime distraction from the Watergate scandal.)