A key to answering that rather enormous question may lie in the secrets that the U.S. government is keeping about Saudi Arabia.
Some have long claimed that what looked like a crime on 9/11 was actually an act of war necessitating the response that has brought violence to an entire region and to this day has U.S. troops killing and dying in Afghanistan and Iraq.
Could diplomacy and the rule of law have been used instead? Could suspects have been brought to trial? Could terrorism have been reduced rather than increased? The argument for those possibilities is strengthened by the fact that the United States has not chosen to attack Saudi Arabia, whose government is probably the region's leading beheader and leading funder of violence.
But what does Saudi Arabia have to do with 9/11? Well, every account of the hijackers has most of them as Saudi. And there are 28 pages of a 9/11 Commission report that President George W. Bush ordered classified 13 years ago.
Senate Intelligence Committee former chair Bob Graham calls Saudi Arabia "a co-conspirator in 911," and insists that the 28 pages back up that claim and should be made public.
Philip Zelikow, chair of the 9/11 Commission, has noted the "likelihood that charities with significant Saudi government sponsorship diverted funds to Al Qaeda."
Zacarias Moussaoui, a former al Qaeda member, has claimed that prominent members of Saudi Arabia's royal family were major donors to al Qaeda in the late 1990s and that he discussed a plan to shoot down Air Force One using a Stinger missile with a staff member at the Saudi Embassy in Washington.
Al Qaeda donors, according to Moussaoui, included Prince Turki al-Faisal, then the Saudi intelligence chief; Prince Bandar Bin Sultan, the longtime Saudi ambassador to the United States; Prince al-Waleed bin Talal, a prominent billionaire investor; and many of the country's leading clerics.
Bombing and invading Iraq has been a horrible policy. Supporting and arming Saudi Arabia is a horrible policy. Confirming Saudi Arabia's role in funding al Qaeda should not become an excuse to bomb Saudi Arabia (of which there's no danger) or for bigotry against Americans of Saudi origin (for which there's no justification).
Rather, confirming that the Saudi government allowed and quite possibly participated in funneling money to al Qaeda should wake everyone up to the fact that wars are optional, not necessary. It might also help us question Saudi pressure on the U.S. government to attack new places: Syria and Iran. And it might increase support for cutting off the flow of U.S. weapons to Saudi Arabia -- a government that takes no second place to ISIS in brutality.
I've often heard that if we could prove that there weren't really any hijackers on 9/11 all support for wars would vanish. One of many hurdles I'm unable to leap to arrive at that position is this one: Why would you invent hijackers to justify a war on Iraq but make the hijackers almost all be Saudi?
However, I think there's a variation that works. If you could prove that Saudi Arabia had more to do with 9/11 than Afghanistan (which had very little to do with it) or Iraq (which had nothing to do with it), then you could point out the U.S. government's incredible but very real restraint as it chooses peace with Saudi Arabia. Then a fundamental point would become obvious: War is not something the U.S. government is forced into, but something it chooses.
That's the key, because if it can choose war with Iran or Syria or Russia, it can also choose peace.