I've always been intrigued by civil lawsuits between individual states; they're like fights between angry neighbors in a subdivision, writ large. On Monday, in a one-sentence opinion, the United States Supreme Court declined to hear a complaint by Nebraska and Oklahoma against Colorado that challenged the Centennial State's proposition legalizing recreational marijuana.
Lawsuits between states are special in American jurisprudence, as they're the only situation where the Supreme Court has original and exclusive jurisdiction (that is, the Court can receive the initial complaint and not just hear the case on appeal). Article III, Section 2 of the Constitution gives the Supreme Court original jurisdiction over "all Cases affecting Ambassadors, other public Ministers and Consuls, and those in which a State shall be Party"; however the Constitution did not explicitly say that this jurisdiction is exclusive, and thus that these cases could not also be brought in lower courts. In the Judiciary Act of 1789, the First Congress established that jurisdiction was not exclusive for disputes between states and citizens or aliens, nor for suits brought by ambassadors or public ministers (it retained exclusive jurisdiction for suits against ambassadors or public ministers). Subsequent revisions pared the scope of exclusivity down until only disputes between the states remained. See 18 U.S.C. § 1251.
In the past, the Court has been solicitous of its obligation to hear cases where it is the exclusive forum for disputes. In the words of Chief Justice John Marshall, "[w]e have no more right to decline the exercise of jurisdiction which is given, than to usurp that which is not given. The one or the other would be treason to the constitution." Cohens v. Virginia, 19 U.S. 264 (1821). However, in more recent centuries the Court increasingly began to defer its jurisdiction, stating that it should only be invoked "sparingly" on the grounds that it lacked expertise in resolving interstate disputes and "so that our increasing duties with the appellate docket will not suffer." Ohio v. Wyandotte Chemicals Corp., 401 U. S. 493, 500–505 (1971); Illinois v. City of Milwaukee, 406 U.S. 91 (1972)). The Court has declined to exercise jurisdiction even in cases where it was presented with a suit between states, which cannot be brought in any other forum. See, e.g., Arizona v. New Mexico, 425 U. S. 794 (1976).
In evolving towards its current posture, the Court has created a fairly high bar to exercise its discretion in state v. state suits. In Massachusetts v. Missouri, 308 U.S. 1 (1939), the Court stated that jurisdiction should be exercised where "recourse to [the Court's] jurisdiction . . . is necessary for the State's protection." In Texas v. New Mexico, 462 U.S. 554, 571 n.18 (1983), the Court stated that “[t]he model case for invocation of this Court’s original jurisdiction is a dispute between States of such seriousness that it would amount to casus belli if the States were fully sovereign.” In other words, unless a dispute is of the type you might expect to send countries to war -- a fight over water rights, the location of a border, etc. -- the Court doesn't want to hear it.
All this is to say that Court's decision not to hear Nebraska and Oklahoma's complaint shouldn't be read as the Court's acceptance of the propriety of Colorado's legalization scheme, or as a challenge to the federal government's prohibition of marijuana. It's more of a punt by the Court, with the justices saying "we don't want to deal with this."
If Nebraska and Oklahoma truly are being harmed by Colorado, there should be a citizen or business that has suffered actual damages that could support a complaint filed in a federal district court; if that case were to eventually reach the Supreme Court, appeals will have narrowed it to something the Court can deal with. Until then, Nebraska and Oklahoma need to just relax -- and I hear there are some new businesses in Colorado that can help.