From a legal point of view, when is marijuana possession a federal offence and when is it a state offence? Presumably if you're caught with weed in DC then it'

From a legal point of view, when is marijuana possession a federal offence and when is it a state offence? Presumably if you're caught with weed in DC then it's federal. But under what circumstances could you be found in possession with marijuana anywhere else and have it be a matter for the federal government, not the state one?