Napoleon created Extraordinary Councils of War in 1812, which were designed to judge general officers, such as Dupont, who surrendered in the field or gave up fortresses without first offering resistance. Dupont was the author of his own misfortune, Cambaceres comments notwithstanding.

And this was a military court which Napoleon had every right to establish in the military justice system.

Did you expect the trial to be long? That wasn't the usual way court-martials were done. Again, see the posting on the military justice system.

And courts-martial was the 'accepted' way of dealing with egregious failure in the field. Great Britain most certainly did it, witness the court-martial of General Murray after his failure at Tarragona in June 1813.