You are wrong. Congress has been given the power to declare war it is true. But one can be at war without or prior to such a declaration recognizing that a state of war exists.
Check out Roosevelt's speech on December 8, 1941.
I believe that I interpret the will of the Congress and of the people when I assert that we will not only defend ourselves to the uttermost but will make it very certain that this form of treachery shall never again endanger us.
Hostilities exist. There is no blinking at the fact that our people, our territory and our interests are in grave danger.
With confidence in our armed forces, with the unbounding determination of our people, we will gain the inevitable triumph. So help us God.
I ask that the Congress declare that since the unprovoked and dastardly attack by Japan on Sunday, Dec. 7, 1941, a state of war has existed between the United States and the Japanese Empire.
Thus the state of war existed *before* Congress made its declaration.
Sorry, friend, but you are the one who is wrong. Article 1, Section 8 of the US Constitution deals with the War and Treaty powers of the nation. Sole power to declare war is invested in the US congress. The President commands the Armed Forces and can do so without a declaration of war—but it is not war.