The Constitution gives three methods for going to war. Formal declaration (and it doesn't say how it has to be worded), punishing 'piracy and offenses against the laws of nations', and putting down rebellions. We did have Congressional authorization for both Iraq and Afghanistan. Funny thing is Paul says that Iraq wasn't legal because the 'authorization for the use of force' in his mind isn't a declaration of war but he voted for a similarly worded authorization for Afghanistan.
Didn’t Congress give authority to Bush to declare war? It seems to me that such an act would have to be done by an amendment to the Constitution to be legal.