To be sure, Congress declared that state policy would disrupt federal law enforcement. It believed the across-the-board ban essential to policing interstate drug trafficking. 21 U.S.C. § 801(6). But as Justice O’Connor points out, Congress presented no evidence in support of its conclusions, which are not so much findings of fact as assertions of power. Ante, at 13—14 (dissenting opinion). Congress cannot define the scope of its own power merely by declaring the necessity of its enactments.
In sum, neither in enacting the CSA nor in defending its application to respondents has the Government offered any obvious reason why banning medical marijuana use is necessary to stem the tide of interstate drug trafficking. Congress’ goal of curtailing the interstate drug trade would not plainly be thwarted if it could not apply the CSA to patients like Monson and Raich. That is, unless Congress’ aim is really to exercise police power of the sort reserved to the States in order to eliminate even the intrastate possession and use of marijuana.
