I bring to your attention the United Nations Participatory Act of 1945, 22 U.S.C. § 287d:
In short, the UNPA gives the president the authority to negotiate peace agreements w/the U.N. Security Counsel in an effort to help establish peace in the face of hostilities abroad provided that a U.N. Resolution has been established concerning such hostilities. However, the President cannot go beyond the limits and scope of the U.N. resolution as mandated.
Bottom Line: The UNPA is that "statutory" authorization per section 2(c)(2) of the War Powers Act.
Without knowing the exact circumstances that prompted GW Bush to take the pre-emptive offensive measures he took against Iran, I can't say for sure if his actions were right or wrong. However, what I will say is if Iran did not pose an imminant threat to U.S. national security interests (i.e., attacking their neighbor causing instability in the region that required U.N./NATO involvement towhich the U.S. became a part of per a U.N. resolution), then I'd be inclined to say yes, GW Bush may have gone a step too far in that regard.
Again, see above as well as
this linked thread that discusses the matter in full detail (far better than I could, I must confess).