When state and federal laws are inconsistent, the default setting is that both are enforced, by their respective enforcers. State and federal governments are independent sovereigns whose laws operate independently on the citizens subject to them. If you have a Texas driver’s license, you can drive on highways in west Texas and run over all the lizards you want—as far as Texas is concerned. But some of those lizards are endangered species under federal law, and running them over is prohibited—as far as Congress is concerned. You can keep your license, but you may get an unpleasant visit from the U.S. Fish and Wildlife Service.

As long as citizens can comply with both sets of laws and Congress can successfully operate its own laws, there should be no conflict. But where there is a conflict, state law must give way, because the Constitution provides that federal law is the supreme law of the land. For example, when state law “stands as an obstacle to the accomplishment and execution of the full purposes and objectives of Congress,” the Supreme Court has said that state law is preempted.

Does state legalization of marijuana interfere with “the full purposes and objectives of Congress” in its own prohibition of marijuana? It does, but only in a very peculiar way. And here’s where the story gets really interesting.

Federal drug-enforcement agencies like the FBI and DEA focus on “big fish” involved in large-scale drug-trafficking. But federal drug enforcement is built atop the edifice of state law enforcement. “It’s a division of labor,” says Percy Martinez, a criminal-defense attorney and former prosecutor in Miami. “States go after the little fish and the feds go after the big fish. But if you don’t go after the little fish, then sometimes you can’t get the big fish.”

Federal law criminalizes all aspects of the illegal drug trade, big and small, but the federal government can’t enforce all of that on its own, because it simply doesn’t have enough money or manpower. State and local law enforcement agencies are the ones with hundreds of thousands of cops on the streets. The DEA’s entire budget would fund perhaps six months of operations for the NYPD.

Federal drug enforcement, in other words, depends on state agencies for “the accomplishment and execution of the full purposes and objectives of Congress.” But states have no obligation to further the purposes and objectives of Congress. Therefore their refusal to do so—or even their adoption of laws that have contrary purposes and objectives—can’t be preempted by federal law.

The complaint filed by Nebraska and Oklahoma alleges that “In passing and enforcing Amendment 64, the State of Colorado has created a dangerous gap in the federal drug control system enacted by the United States Congress.” That’s clearly wrong. Amendment 64 may have exposed a dangerous gap in the federal drug-control system. But the gap was created by Congress, not Colorado. As the Justice Department memo explains, in most cases “the federal government has traditionally relied on states and local law enforcement agencies to address marijuana activity through enforcement of their own narcotics laws.” Congress in effect chose to depend on the states, by passing laws more sweeping than it can enforce on its own. States can’t be penalized for that under the doctrine of preemption.