Democrats in California believe they can impose their laws on the rest of the country, and they even think they can ignore the Constitution when it conflicts with their progressive policies. Credit to the Justice Department for attempting to make clear that California isn’t a separate nation under the law, and that climate change isn’t a license for a state to conduct its own foreign policy.

Last week the Justice Department sued California for entering a cap-and-trade agreement with Canada’s Quebec province. Since 2013 California...