Without authorization from Congress, American presidents can only start unconstitutional wars, even if they are motivated by good intentions.
A president asserts the prerogative to take the nation into a state of war, completely on his own authority as “Commander in Chief” of the nation’s armed forces, in a situation not involving an attack upon the United States or its citizens or armed forces stationed abroad, and not involving an emergency situation in which there is no time to consult with Congress.
Congress counters by insisting that the power “to declare War” is vested in Congress, not the president, and that only Congress can authorize the use of offensive military force against an enemy nation or power—and then adding its views as to exactly how much force should be used, where, when, by whom, and with what goals and time limits.
The president welcomes—invites—such congressional “support” for his claimed right to make war on his own, but continues to maintain that, constitutionally, he does not need it. Congress meets, deliberates, debates, argues. The president hems and haws, blusters and postures. Meanwhile, the putative enemy prepares, thankful for the additional time in which to act.
Sound familiar? It should. This scenario has happened numerous times in modern history. It is happening yet again, with respect to proposed US military intervention in Syria.