Any way he did it he did it without Congress.
Any way he did it he did it without Congress. To do otherwise might clash with and draw attention to the way he declared the "war" in the first place.
Nowadays, all Congress gets to rubberstamp is Uncle Sam's signature at the bottom of the blank checks written to cover the cost of whatever "moral" war needs fighting at this or that nexus of oil or drugs.
For all of our Constitutional scholars here: This order relates to the trial of enemies of the United States, fighting without uniform and attacking civilian and non-military targets, or giving aid and comfort to the same, in a time of war. When Congress passed the resolution after 9/11, they acknowledged that a state of armed conflict existed with the terrorists. Once that was done, the Constitutional authority to deal with these enemies was passed to the commander in chief of the armed forces. Military Tribunals have been tested in the courts and have been found constitutional for this purpose. So all three branches of the government have been involved according to their constitutional role. Bush has the authority to establish these tribunals, as it is a military matter, and Congress has no constitutional authority to interfere with his duties as commander in chief.
If Bush were to use tribunals to try citizens or to punish other crimes, then you might have some reason to object. But the fact of the matter is, the tribunals are legal, constitutional, a common tool in every war, and even acknowledged in international law. (As a matter of fact, combatants not in uniform can be executed without benefit of even a trial before a tribunal, according to the Geneva Conventions.)