With regard to enforcing sanctions, the last eight years, if not previous president, have opened the door to selective enforcement. Congress did practically nothing over it then when it clearly violated their intent and have not made a real peep this time. Under the previous administration congress blamed the lack of enforcement on the executive agencies involved. International sanctions would involve any number of them so perhaps it is too much trouble and the players are too Establishment to risk blaming in order to get to Trump.
The president can not over rule congress without constitutional grounds. Under the question of sanctions, that would be difficult. Congress’s actual constitutional powers are primarily external. International trade, commerce, and relations are clearly theirs, so there is no actual constitutional ground not to enforce their will. It can only be called a policy dispute.
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