But it is not only a problem of moral, but as well of pratical issues.

• The London terror attack was done by people born in the UK and living all their lives in the UK.
  
• There are lots of countries which would provide enough lack of rule of law to be used by terrorists as base. The 'we will hunt the terrorists where they are wherever they hide' is impossible. Even if the resources would be there to attack every country. For a pseudomoralic justification you need as well nationbuilding. There may be resources for one or two big countries nation building, but not for a dozen.
  
• The precision of war is lousy, you kill a lot more people than the few, who would likely engage in terror attacks. This killings may increase the propensity to do terror attacks by the remaining people - and gives them justification
  
• There is zero possibility to control success. Nobody knows, if the war in Afghanistan has increased or reduced the likelyhood of further terror attacks. It is impossible to find out.
  
• the war in Afghanistan reduces available resources. Al Kaida has attacked the US, because the US is in the middle east. Nobody who has openly looked to the motives of Al Kaida is denying that. The most important reason to be there is the stability of the major oil suppliers. With less expenditures for war, the US could have been much more energy efficient and independent by now.
  
• what is the cost in whatsoever of another terror attack in the US compared with wars, in $, human live, international respect, future opportunities...

Yes, you don't want a bankrobber to get away, but even in the US the police will not kill a whole village to get one bankrobber, when they e.g. have proofs, that one of the people in the village must be the bankrobber, but they don't know who.

Der Amerikaner ist die Orchidee unter den Menschen
Volker Pispers