So all the president needs to do is declare he is acting as Commander In Chief, and no matter what, he is correct? We just let him evoke that power to whatever end, with no oversight or restraint? Or do you mean something a bit less than dictatorship?
The president answers to the electorate every 4 years. Wrongs are generally righted at those times. In the interism, gross misbehavior can result in an impeachment and removal from office.
In the meantime, if the President says "I tapped the line of terrorist sympathizers because of an immediate need regarding those who murdered 3000 of our citizens on 9/11, and with whom we are officially in conflict by act of Congress." then I'm going to believe him.
If he says, "I'm requiring as commander in chief that all commercial enterprises to send me a thousand bucks a year.", then I won't.
I'm able to tell the difference.