Is cannabis legal in all state of the U.S.?

Technically marijuana is illegal in all 50 states under federal law. The local police may not chose to enforce the law but any federal DEA agent can make a drug arrest, even in a state that "allows it".

Marijuana is classified by the U.S. government as a schedule 1 drug which means the U.S. government does not believe marijuana has any medical value and is likely to be abused as a drug.