The 9/11 plot was formulated, researched, planned out in many nations; Afghanistan, Pakistan, Syria, Iraq, Germany, England, Spain, and the U.S.
When other nations support terrorists, provide them money, training, transportation, communication, etc so they can plot and pull off an attack on American soil, then it's our duty to "interfere in their affairs."
War is not longer just 1 or 2 nations against others... this is world-wide. So we'll stick our noses anywhere we must to keep our people safe.
George Bush: Saving your ass... like it or not!
I'd prefer our troops at home as well, but's more xenophobic than we, the world power, should be. Rogue nations are less impressed when all our troops and carriers are nowhere in their vicinty than when they're only a few day's sail away.