One of the several failures of the Articles of Confederation was the incapacity of the central government to deal with trade disputes among the states. The Constitution resolved this problem by empowering the federal government to regulate interstate commerce. It has since become a basic principle of American federalism that a state may not regulate actions in other states or impede the interstate flow of goods based on out-of-state conduct (rather than on the features of the goods themselves). That principle was axiomatic until the U.S. Court of Appeals for the Ninth Circuit upheld one particular extra-territorial California regulation. California recently established a Low Carbon Fuel Standard (“LCFS”) that attempts to rate the “carbon intensity” of liquid fuels, so that carbon emissions can be reduced in the Golden State. California considers not only the carbon emissions from the fuel itself being burnt, however, but also the entire “lifetime” of the fuel, including its manufacture and transportation. This has led to complaints from Midwestern ethanol producers, whose product—which is in all other ways identical to California-produced ethanol—being severely disadvantaged in California’s liquid fuel markets, simply because it comes from further away. Groups representing farmers and fuel manufacturers sued, arguing that the LCFS constitutes a clear violation of the Commerce Clause (the Article I federal power to regulate interstate commerce) by discriminating against interstate commerce and allowing California to regulate conduct occurring wholly outside of its borders. The Ninth Circuit recently upheld the LCFS, finding the regulation permissible because its purpose was primarily environmental and not economic protectionism (although judges dissenting from the court’s denial of rehearing pointed out that this is the wrong standard to apply). The farmers and fuel manufacturer groups have now submitted a petition to have their case heard by the Supreme Court. Cato has joined the Pacific Legal Foundation, National Federation of Independent Business, Reason Foundation, California Manufacturers & Technology Association, and the Energy & Environmental Legal Institute on an amicus brief supporting the petition. We argue that the decision below provides a template for other states to follow should they want to evade Supreme Court precedents barring obstruction of interstate commerce and extraterritorial regulation. As the Founders fully recognized, ensuring the free flow of commerce among the states is vital to the wellbeing of the nation, and California’s actions—and the Ninth Circuit’s endorsement of them—threaten to clog up that flow. Not only does the lower court’s ruling allow California to throw national fuel markets into disarray, it invites other states to destabilize interstate markets and incite domestic trade disputes—precisely the type of uncooperative behavior the Constitution was designed to prevent.