The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.
The Tenth Amendment, which makes explicit the idea that the federal government is limited only to the powers granted in the Constitution is generally recognized to be a truism. In United States v. Sprague (1931) the Supreme Court noted that the amendment "added nothing to the [Constitution] as originally ratified."
From time to time states and local governments have attempted to assert exemption from various federal regulations, especially in the areas of labor and environmental controls, using the Tenth Amendment as a basis for their claim. An often-repeated quote, from United States v. Darby, 312 U.S. 100, 124 (1941), reads as follows:
The amendment states but a truism that all is retained which has not been surrendered. There is nothing in the history of its adoption to suggest that it was more than declaratory of the relationship between the national and state governments as it had been established by the Constitution before the amendment or that its purpose was other than to allay fears that the new national government might seek to exercise powers not granted, and that the states might not be able to exercise fully their reserved powers.
The Tenth Amendment makes explicit what had before only been implied. It is also worth keeping in mind that the reserved powers mentioned in the Tenth Amendment are not the only powers that a state governments possess. They additionally have what are called concurrent powers, which are held by both the states and the federal government. However, Congress may be able in some instances to preempt those concurrent state powers.
Under the premise that the Constitution splits all federal powers into two groups, i.e., those of Constitutional level and those of lesser stature authorized by the Necessary and Proper Clause, there is also a minority opinion that argues that the Tenth Amendment, being of the higher Constitutional level, prohibits all Constitutional level powers not authorized. By definition, any and all powers mentioned in the Constitution are of the Constitutional level, and all powers that are not mentioned are of lesser stature. Since mentioned powers are mostly prohibitions on the states, this interpretation argues that the Tenth Amendment makes those prohibitions effective on the federal government as well, except in cases where specific authorization is granted. This interpretation is contrary to the widespread use of the Necessary and Proper Clause to authorize the use of Constitutional level powers that are mentioned but not authorized.
In the twentieth century, complex economic challenges of the Great Depression triggered a reevaluation in both Congress and the Supreme Court of the use of Commerce Clause powers to maintain a strong national economy.
In Wickard v. Filburn (1942), in the context of the Second World War, the Court ruled that federal regulations of wheat production could constitutionally be applied to wheat grown for "home consumption" on a farm--that is, wheat grown to be fed to farm animals or otherwise consumed on the farm. The rationale was that a farmer's growing "his own wheat" can have a substantial cumulative effect on interstate commerce, because if all farmers were to exceed their production quotas, a significant amount of wheat would either not be sold on the market or would be bought from other producers. Hence, in the aggregate, if farmers were allowed to consume their own wheat, it would affect the interstate market in wheat.
In Garcia v. San Antonio Metropolitan Transit Authority (1985), the Court changed the analytic framework to be applied in Tenth Amendment cases. Prior to the Garcia decision, the determination of whether there was state immunity from federal regulation turned on whether the state activity was "traditional" for or "integral" to the state government. The Court noted that this analysis was "unsound in principle and unworkable in practice," and rejected it without providing a replacement. The Court's holding declined to set any formula to provide guidance in future cases. Instead, it simply held "...we need go no further than to state that we perceive nothing in the overtime and minimum-wage requirements of the FLSA ... that is destructive of state sovereignty or violative of any constitutional provision." It left to future courts how best to determine when a particular federal regulation may be "destructive of state sovereignty or violative of any constitutional provision."
In United States v. Lopez , a federal law mandating a "gun-free zone" on and around public school campuses was struck down because, the Supreme Court ruled, there was no clause in the Constitution authorizing it. This was the first modern Supreme Court opinion to limit the government's power under the Commerce Clause. The opinion did not mention the Tenth Amendment, and the Court's 1985 Garcia opinion remains the controlling authority on that subject.
Most recently, the Commerce Clause was cited in the 2005 decision Gonzales v. Raich. In this case, a California woman sued the Drug Enforcement Administration after her medical marijuana crop was seized and destroyed by Federal agents. Medical marijuana was explicitly made legal under California state law by Proposition 215; however, marijuana is prohibited at the federal level by the Controlled Substances Act. Even though the woman grew the marijuana strictly for her own consumption and never sold any, the Supreme Court stated that growing one's own marijuana affects the interstate market of marijuana, citing the Wickard v. Filburn decision. The theory was that the marijuana could enter the stream of interstate commerce, even if it clearly wasn't grown for that purpose and it was unlikely ever to happen. It therefore ruled that this practice may be regulated by the federal government under the authority of the Commerce Clause.
For this reason, Congress often seeks to exercise its powers by offering or encouraging the States to implement national programs consistent with national minimum standards. The mechanisms are discussed in the article on cooperative federalism. One example of the exercise of this device was to condition allocation of federal funding where certain state laws do not conform to federal guidelines. For example, federal educational funds may not be accepted without implementation of special education programs in compliance with IDEA. Similarly, the nationwide state 55 mph (90 km/h) speed limit and the nationwide state 21-year drinking age were imposed through this method; the states would lose highway funding if they refused to pass such laws. See e.g. South Dakota v. Dole, 483 U.S. 203 (1987).