His argument is, Bin Laden has assumed that, because we backed down, we were weak and would remain so. There is more to it than that, of course. But the perception of our weakness -- valid or not, is a large part of our problem since even before Vietnam. It certainly does go back more thana half-century. To wit, Bill Mauldin's 1961 cartoon and comment from What's Got Your Back Up?
There are (at least) two categories of error in foreign policy. One is to commit egregious acts against a people such that they rise up against you.
The other is to show weakness, such that they think that if they can hurt you badly enough, you'll give up, and give in to their demands, no matter how outrageous and unreasonable they may be.
While we've done more than we should have of both over the last...well...half century, if not longer, the latter is the major reason that we are currently under siege (at least metaphorically, if not literally).
Yes, bin Laden whined about the "occupation of Arabia" during and after the first Gulf War. And the Arabs continually whine about the oppression of the "Palestinians" by the "Zionists" (see, I can use scare quotes just as well as Reuters, except...well, mine are actually accurate).
But the real reason for the war we're in can be found in the part of bin Laden's speech about the "weak horse" and the "strong horse."
More to the point is the comment Mauldin includes beneath the image:
Totalitarian leaders do their worrying in private and always show a resolute face to the world. It is hard for them to understand societies which struggle openly with decisions, and it must be very easy for dictators to make the dangerous mistake of confusing soul-searching with flabbiness.
By and large, since the 1970s, we have been a flabby nation -- especially during terms when military-loathing Democrats have had greater power in Washington. Nevertheless, historically and currently, this nation does not tolerate that sort of self-destructive behavior too often or for too long. Elections come, truth about real threats is recognized, and would-be Jimmy Carters fail swiftly.
I hope I'm not mistaken about my fellow countrymen (not to mention other free peoples). I would hate to see Robert Heinlein proven wrong on his 100th birthday.