I don't think terrorism would be a reasonable threat if the country was reasonable. The US, as well as the western world, has interfered with regions associated with terrorism for the last century, pitting people against people, arming specific groups, etc. And we did so not for some moral reason that they may identify with, we did it solely for economic gain.
There is a reason the Twin Towers were attacked: they were a system of capitalistic greed.
I believe we have a large military because it is economically profitable for some individuals who happen to have a lot of influence (because, of course, of their money).