Simply increasing its popular support by showing the rightness of our objectives from the start is broadly (but immeasurably) helpful. Mostly through politics, admittedly, but that's how real-world wheels begin turning.
Too late - at this point in the war most in the U.S. and the west could not care the least about any smoking gun that might turn up. Want to turn public opinion? Show progress in Iraq by conducting a successful Counterinsurgency campaign. Hint - hint - COIN is typically 20% military and 80% political...