Whenever somebody makes the agument that "we need to do X or else we'll look weak", I usually discount the argument pretty substantially. That's only one of the many funny things that you can read in this article that Rob deals with quite handily. As Rob mentions, the lamentations over the US defeat in Vietnam spurring the Soviet invasion of Afghanistan truly is amusing. While there's a number of reasons to be opposed to foreign adventurism by any country, including the US, there's very little reason from a US power perspective to say that the invasion of Afghanistan was a bad thing*. It hurt the USSR enormously, the Soviet military never really recovered, and it's one of the signal failures of governance that precipitated the collapse of Communism. Good things on their own, but especially good things for the US.
If the US departure of Vietnam spurred the Soviet invasion of Afghanistan, then for the love of God let's leave Iraq, pronto. Maybe we can get the Iranians to invade when we leave -- that worked out so well for the Ayatollahs last time.
*And yes, that includes the external costs of 9/11. Do you think a single American, if asked in 1980, would have turned down a deal where the Soviet Union ceases to exist, but America loses 3,000 civilian lives?