An interesting point, but the critics seem to forget that Congress--altough it did not declare war--did authorize the use of force. It can be argued (my guess is successfully as in the case of Lincoln) that in such cases, the President (as Commander in Chief of the Armed Forces) is authorized to declare states of rebellion and empower tribunals under his authority.
Ah! Force bills! of course. These have often been issued by Congress in peacetime. In a real sense in the old days they bypassed the President. It was a case of congress exercising military power on it's own. In Lincoln's day, when Lincoln took his actions when Congress was out of town, upon his return he went back to them to have them sanctioned.
The President was only the Commander in Chief during war. In peace time he was not. As part of the imperial presidency he is essentially now full time generalissimo. In the old Republic, the President tipped his hat when the military saluted. I like that so much better.