Conservatism: fear writ large
I've had this idea in my mind for a couple weeks now, and now after spending several days with a very conservative family member, I'm convinced: the driving force behind American conservative foreign (and occasionally domestic) policy is the overwhelming fear that America is actually very weak.
A bunch of guys hiding in caves are really such a threat to America that the US military has to have a presence in the majority of countries, or we might be toppled. The "Ground Zero mosque" has to be defeated because if "the Muslims" get this foothold, they really might introduce Sharia law into America. This is not a joke.
To them, anyway.