"I have spent some time and thought about it, as you asked me to do, and unless we know more about how a terrorist organization operates, I don't think the higher threshold you described necessarily eliminates the possibility that the U.S. presence in Iraq is preventing terrorist organizations there from conducting overseas attacks by drawing down their resources."
I have explained that.
Unless your scenario is nothing more than crossing the border and throwing a pipe bomb, there simply are not enough terrorists in the available pool to conduct frequent operations in the USofA.
Now, unless you believe that al Queda is offering English language classes, you're simply repeating yourself.
If they aren't expanding the pool, they will forever be limited that extremely small pool.
You can argue with that all you want, but it is not going to change.
Instead, a better use of your time would be to look at the problem from al Queda's point of view. You can't really hold English language classes.
But you can support a few individuals who would live in the USofA and attempt to radicalize US citizens who already know English and our customs and such.
Look at the attacks in England. Tell me about the attackers. Who were they? Where did they live? What language(s) did they speak? Where did they go to school? Where did they get the weapons?
"He may believe exactly what I said: by being there, we're keeping them from getting over the threshold that keeps them from attacking the U.S. I don't think the Clarke article works this issue through."
Belief is useless. Facts are what matter.
"The President might be wrong--in fact, I think a withdrawal would likely result in greater foreign attacks, specifically on oil fields--but his beliefs might have nothing at all to do with chivalry."
Yes, I understand what you believe. But you have not stated any facts to support your beliefs. Nor have you explained why those facts support your beliefs.
The cost to move a terrorist to the USofA is almost nothing. A ticket to Mexico from any near by country. A map, a canteen of water and a compass and he's in the USofA.
I've checked the sites and a one way ticket from Turkey to Mexico is about $1,000.
So your position pretty much depends upon them not being able to scrape up $1,000 per terrorist because of the war ... but if the war were to end, they'd be able to scrape up that much money.