A law requiring a Declaration before hostilities would not work for a number of reasons. One example:
We come under nuclear attack (or any other attack that prevents Congress from convening). The President cannot employ weapons to defend the US until a Declaration of war is made. US (nuclear) weapons are destroyed on the ground before Congress can convene, and we are reduced to rubble.
We come under nuclear attack (or any other attack that prevents Congress from convening). The President cannot employ weapons to defend the US until a Declaration of war is made. US (nuclear) weapons are destroyed on the ground before Congress can convene, and we are reduced to rubble.