Subnational units of government are critical actors in the U.S. federalist scheme of regulation. It was the original 13 colonies/states which were the core of the American experiment, and banded together as a nation for common defense and commerce after fighting for independence from the United Kingdom. The Constitution vested in the new federal government the treaty and war powers, as well as powers over interstate commerce.
"Subnational Discretion Mediating New Climate Regulatory Challenges,"
San Diego Journal of Climate & Energy Law: Vol. 7
, Article 3.
Available at: http://digital.sandiego.edu/jcel/vol7/iss1/3