Even the conservative YAL doesn't buy into the notion that the war is justifiable because of the actions of a group of people who were at one time financed by the same political party who caused the war today:
Make the Right Choice, Mr. President | Young Americans for Liberty
That's all well and good, but Bush didn't cause the war.
The war was caused by a direct attack on US soil.
You can try and spin the fact that at one point, Osama was a CIA bag man all you want. But the fact is, just because a son turns on the parents that made him, doesn't make the parents the cause.
The attack on the US was formulated in Afghanistan, the perpetrators protected and funded by the Gov't of said country. Then allowed to act out side that countries boarders, then refused to hand the perpetrators over. Thus making them culpable.
The alternative is, the Taliban simply allowed them to concoct their plan, within their borders. When the plan came to fruition outside their borders, that made Afghanistan a failed state.
In either scenario, the outcome is invasion. Either for retaliation for said attack, or to secure a failed state.