The absence of congressional authorization may not stop military action against Iran—but it likely means that any operation undertaken would be relatively small in scale.
In the aftermath of the September 11th attacks, Congress voted to authorize military force against against those who had “planned, authorized, committed, or aided the terrorist attacks” or who "harbored such organizations or persons, in order to prevent any future acts of international terrorism against the United States. . . ." More than a decade later, the United States continues to conduct military operations against Al Qaeda and affiliated groups—including, somewhat surprisingly, against the Islamic State—pursuant to this 2001 authorization. In the spring of 2015, despite claiming that a new AUMF for airstrikes and limited operations in Syria and Iraq was not legally required, the Obama Administration presented draft language to Congress for a new AUMF. Riven by disagreements over details, Congress has thus far refused to support the President’s proposal.