Marijuana is classified under federal law as an illegal drug. On what enumerated power would Congress have relied when it enacted the federal statute that outlaws marijuana and other specified drugs? 

A number of states have legalized marijuana possession and use up to certain levels designated in their laws. Several other states have legalized marijuana possession and use for medicinal purposes, but for those purposes only. As a constitutional matter, could the federal government if it were so inclined adopt an aggressive enforcement posture in which it would override the state laws to the contrary? If so, on what constitutional basis? If not, why not?