From Webb's speech introducing the legislation:
"Mr. President, during our recent hearings on the Senate Foreign Relations Committee, I asked both the Secretary of State and the Deputy Secretary of State for clarification of this paragraph in the President's signing statement. My question was whether this Administration believes it has the authority to conduct unilateral military operations against Iran in the absence of a direct attack or a compelling, immediate threat without the consent of the Congress. Both wrote me lengthy letters in reply, but neither could give me a clear responses. The situation that we now face is that the Administration repeatedly states that it seeks no war with Iran, at the same time it claims the authority to begin one, and at the same time it continues a military buildup in the region.
"The legislation I introduce today is intended to clarify this ambiguity. In so doing, the Congress will be properly restating its constitutional relationship with the executive branch. The Congress will be reinstituting its historical role as it relates to the conduct of foreign policy. And the Congress will be reassuring the American people that there will be no more shooting from the hip when it comes to the gravely serious question of when we send our military people into harm's way.
Go read the whole speech -- oustanding.