Article I, Section 8 of the Constitution grans the authority to declare war to Congress. However, Congress has not voted to declare war on any country since 1941, our entry into World War II.
A Congressional declaration of war is not just a bureaucratic exercise, it's an opportunity for the voice of the American people to be heard, through their elected representatives, on the national stage. If North Korea initiates hostilities, the resulting conflict will likely be a long and bloody one; because of this, Americans deserve to have their voices heard, even if those voices run counter to the Administration's wishes. If we can vote to enter World War II (which, incidentally, was not a unanimous decision), we can vote on this.