Columnnist Gary Young used a thought provoking analogy in his comment piece in today's Guardian:

To say that terrorists would have targeted us even if we hadn't gone into Iraq is a bit like a smoker justifying their habit by saying, "I could get run over crossing the street tomorrow." True, but the certain health risks of cigarettes are more akin to playing chicken on a four-lane highway. They have the effect of bringing that fatal, fateful day much closer than it might otherwise be.

And earlier in the same piece he draws an even more powerful parallel:

We know what took place. A group of people, with no regard for law, order or our way of life, came to our city and trashed it. With scant regard for human life or political consequences, employing violence as their sole instrument of persuasion, they slaughtered innocent people indiscriminately. They left us feeling unified in our pain and resolute in our convictions, effectively creating a community where one previously did not exist. With the killers probably still at large there is no civil liberty so vital that some would not surrender it in pursuit of them and no punishment too harsh that some might not sanction if we found them.

The trouble is there is nothing in the last paragraph that could not just as easily be said from Falluja as it could from London....