However, US war mongering is more like sports - and permeated with sport language and imagery - supporting your team, kicking ass etc. I do not think that an average Joe Schmoe realizes how horrifying the war is, mainly because so few actually experienced it (unlike Europeans or Asians). They simply treat it was if it was a ball game - with a lot of help of government propaganda. However, I do not think the support for war would disappear very quickly, if the graphic images of carnage were broadcasted on evening news.
I think that the general boorishness is - as most other social phenomena - a combination of several different factors. The social isolation, exacerbated by suburbia, and the diminishing role of public life is but one of them. Add to ubiquity of passive entertainment (TV) that further erodes the need for social skills - people do not need to interact t with one another (which requires skills) but passively watch tv and comment on it.
Another aspect is the consumer culture promoted by marketing that develops a sense of entitlement and "having it your way" which the marketers promise to every consumer buying their product - and by making product to satisfy ever possible whim. People have been conditioned to get what they want, and if they do not get it, they have no skills to negotiate it, so they get upset and violent.
All that takes place in the context of anti-intellectual populism and individualism that was hostile to high culture civility and manners - lamented by writers like HL Mencken or Richard Hofstadter. Boorishness was not only ubiquitous but also considered a sign of "manly folksiness" and a virtue vis a vis emasculated manners and culture.
With that history and with colonization of this society by marketers - it is amazing that boorishness is not even more widely spread.
Wojtek