"I do wonder what the outcome would have been if we shot down all the planes before they hit their targets?"

Yeah, that is one to ponder, Ron. It's unlikely that action would have been taken though, until at least one plane had deliberately struck a target and it was realized by the military what was going on. I mean, look, I wouldn't shoot down a hijacked airliner until I had very strong reasons to believe that it was the ONLY course of action possible to prevent many more lives being lost than just those on that airplane. However, suppose it had been done. Suppose they had shot down all those airliners...or all except one of them.

I think the outcome would have been absolute shock and horror on the part of the whole country at the loss of civilian life (which would have certainly included anger at the US military for possibly "jumping the gun")...that it would, however, very soon have been blamed on Osama Bin Laden and Al Queda...that public anger would have then have mostly shifted to them as the guilty parties...and that the attack on Afghanistan would have gone forward in exactly the same way as it did. And later the attack on Iraq as well.

No appreciable difference to foreign policy, but a more than appreciable difference to the WTC buildings, the Pentagon, and almost 3,000 dead Americans who would not have died on that day.

A preferable result, in other words, but still not too good on the whole.

Mind you, I think they had other reasons to knock down the WTC (financial reasons), and I think they had reasons to have as big and publicly shocking an event as possible, something that could be seen on live TV. You can't do that nearly as effectively by just shooting down some airliners.

It's like...suppose (just for the sake of argument) that the Americans on Dec 7 '41 had intercepted the oncoming Japanese carrier force (or its aircraft in the air) and destroyed them before they had a chance to hit Pearl Harbour? What if? Would it have worked as well as a psychological motivator to mobilize an entire nation as the historical Japanese attack on the harbour and the spectacular sinking of the Arizona and those other ships did?

Hell no. You need a real sense of damage to get people damn angry. That requires suffering a very visible surprise attack by someone and taking significant losses. FDR needed that report from that radar station on Oahu to be ignored or discounted when they saw the Japanese planes coming in! I'm not saying that necessarily proves it was planned that way...I'm just pointing out how these events work psychologically, and what is required to enrage an entire nation and make them forget about anything except striking back.

The Neocons had a more difficult proposition on their hands than FDR in '41. He knew he could provoke another nation-state into striking the first blow militarily...but there was no way that the USA could get Afghanistan or Iraq to attack the USA like that. It was not even logistically possible. So they had a more subtle problem on their hands if they wanted a war, and it required the existence of shadowy undercover killers to do it (whether or not they were foreign or domestic-based). If Osama didn't exist, they would have had to invent him... ;-) If Saddam didn't exist, they might have had to invent him too. Ditto for Ahmadinejad.