system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What gives the federal government the right to ban marijuana? And why can states violate that? [Serious]

I keep hearing about how the federal gov't bans all pot. But states still legalize medical marijuana, and in the case of Colorado, even recreational pot. So why can states violate the federal law? Also, if it took a constitutional amendment to ban alcohol at a federal level, why can they just decide to ban pot?
gpt
The Commerce Clause of the Constitution gives Congress the authority to regulate items that have a high probability of being trafficked across state borders. 

States that have chosen to legalize marijuana are simply choosing not to enforce federal marijuana laws and have gotten rid of most of their own criminal & civil penalties on the sale and trafficking of the drug. If the federal government wants to enforce those laws they now must carry out enforcement itself. 

Prohibition was put through as a Constitutional Amendment because proponents felt it would be a more permanent change than a simple law that could easily be overturned. Remember when Prohibition passed no Amendment had ever been overturned at that point.