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An example of the federal government acting upon the assumption that all federal law is supreme over State law is the medical marijuana laws in California. Though I do not agree with the legalization of marijuana, even for medicinal purposes, the actual constitutional legality of the issue illustrates my point quite well.
California passed a law legalizing marijuana for medicinal purposes, but federal law has marijuana as being illegal in all applications. Therefore, using the authority of the federal government based on the Supremacy Clause, federal agents (or at least until recently) have been raiding and shutting down medical marijuana labs in California. However, there is no place in the U.S. Constitution that gives the federal government the authority to regulate drugs, nor has there been an amendment passed to grant that authority to the federal government, therefore the raids on medical Marijuana labs in California are unconstitutional actions by the federal government.
The Supremacy Clause applies only to federal laws that are constitutionally authorized. Therefore, federal drug laws are unconstitutional. As a result, California's medical marijuana laws are constitutional because they are not contrary to any constitutionally authorized federal laws.