William Raspberry realizes the importance to the anti-war movement of refuting the claims that the IWR vote was vote to go to war. He has an explanation which includes an good analogy:
Except they didn’t really vote us into war. As I recall, that vote authorizing the president to use force against Iraq was analogous to a trade union’s strike vote. When negotiations bog down, union leaders often will ask their members for a resolution authorizing a strike. For members to refuse such a vote would cripple their own leadership. But to grant it is not the same as ending negotiations and launching a strike. It is a way of steeling the leadership, giving it a powerful negotiating tool.
(Hat tip to Upper Left for finding this op-ed).