Going to war to prevent war. Most of us would rather only have to watch people dying, being shot, being tortured and things exploding on the news rather than it happen on their bus ride or tube journey.
Terrorist groups such as Al Quada operate in many, many countries. So, occupying Afghanistan isn't really making much difference. In my opinion, with only several thousand troops it's a waste of money and a losing battle against a backwards and corrupt regime.