This is untrue. Congress has the power to declare war, but that does not mean that we can be at war ONLY if Congress declares war.
At the time the Constitution was written, international law held that a state of war could exist if: 1) one's country was attacked; 2) one's country attacked another.
Only a country waging an aggressive war was expected to issue a declaration of war. This remained true until recent decades.
Waging aggressive war is NOW illegal (which it was not in 1789).
Thus, Congress has not declared war in recent decades, and it WILL NOT declare war--but we are at war nevertheless, because we have been attacked.
Yes and no. Obviously a state of war can objectively exist whether or not Congress passes a resolution declaring war. But the purpose of that provision in the Constitution was to invest the war-making power in Congress rather than the President. It is eminently sensible that it should take more than the whim of one man (the President) to drag this country into a war, and therefore it is the responsibility of Congress to make such a decision.
Obviously there are circumstances where emergency action must be taken in self-defense, and insufficient time is available to seek Congressional approval. An example from decades past was the threat of a massive nuclear attack by Soviet Union ICBMs: It was recognized that the President might have to order a response before the enemy wave of ICBMs struck and destroyed our own nuclear assets. Complex systems and procedures were authorized ahead of time by Congress in an attempt to deal with such situations.
But when adequate time is available, as was the case after the 9-11 attacks, there is no reason not to utilize the proper Constitutional procedures by having Congress pass an official declaration of war. There is some consolation in the fact that Congress did approve military action, so that the President was not in fact acting unilaterally. But why not go all the way and actually declare war as specified in the Constitution? That would be so much cleaner, and would eliminate any possible objections as to whether our nation was really at war and should be acting accordingly.
I would be interested in your comments.